Sam was one of four changes made from the game in Paisley. Shay Logan came back in at right-back to replace Zak Vyner, Jon Gallagher was in for Scott Wright and Niall McGinn started instead of Connor McLennan, meaning the Dons had the same starting XI as in Croatia last Thursday. Rijeka made one change from their starting line-up last Thursday, Raspopovic replacing Tomecak.
Cosgrove's fitness was tested early on by three meaty challenges in the first three minutes, but he shook them off without any concerns. Rijeka threatened first with a mazy run by Acosty, Andy Considine across to snuff out the danger with a well timed tackle. That was to be a warning of things to come.
Despite Rijeka being so physical early on, it was Funso Ojo who first went into the book after six minutes, one of a string of poor decisions by the whistler, though he was not helped by the visiting players.
The Dons were put deep in trouble after just 10 minutes, a quick break by Acosty from a Dons corner allowing him to run 40 yards and then play the ball back to Kvrzic. His curling cross into the near post found Loncar, running into space to apply a neat finish inside the near post from close range when unmarked.
Acosty was the thorn in the Dons' side and another strong run down their right finished with him setting up Halilovic from 25 yards, his shot flying well over the bar after 12 minutes as Aberdeen struggled to get anything going, not finding any quality to their passing.
Things went from bad to worse on 20 minutes when Ojo got his second yellow card for just his second tackle of the evening, Capan making a meal of his challenge, Joe Lewis also booked for his protests by the card happy referee.
In the 25th minute, Jon Gallagher took a real whack to the head from a clearance, the on loan Irishman needing some treatment before he was able to carry on. A minute later, Cosgrove saw yellow for a sliding challenge, perhaps the first legitimate booking of the evening.
Lewis Ferguson produced some real magic in the 29th minute, dancing past several challenges before heading into the box and being knocked off the ball, Aberdeen's unconvincing penalty claims ignored by the referee.
Aberdeen were hit by the sucker punch again in the 32nd minute, another electric break from Acosty seeing him gallop into the Aberdeen half before Gallagher made a challenge. Typical of Aberdeen's luck, the loose ball went straight to Halilovic and his clipped cross to the back post was met by Colak who headed through Lewis' legs and in to seal Rijeka's progress.
Dean Campbell was introduced to the action to plug the gap left by Ojo on 35 minutes, replacing Niall McGinn as the Dons looked to stem the Rijeka tide.
A Ryan Hedges corner from the left on 37 minutes saw Cosgrove penalised again for a challenge on Raspopovic who rolled himself virtually out of the penalty area just to make his point. A spell of Dons pressure in the 40th minute couldn't yield a chance but at least it was more positive from the home side.
Terrific play and persistence from Gallagher on the left finally saw the Dons working the goalkeeper in the 44th minute, the ball eventually worked to Hedges and his 25 yarder was pushed away low down to his left by Prskalo. Moments later, blatant handball from Acosty gave the Dons a free-kick but once again, the referee wasn't interested in employing the notebook for a Rijeka name. With that, a miserable first half was at its end.
HALF TIME: Aberdeen 0 HNK Rijeka 2
Good play from Loncar opened the second half, darting into the box before throwing himself to the ground, the referee running across to him to make the point it wasn't a penalty, but taking no action for simulation.
With the tie long gone, Cosgrove, on a booking, was given a rest in the 53rd minute, Curtis Main joining the action in his place.
With the Dons' ten men tiring, Rijeka were able to keep possession and move the hosts around with ease and on 54 minutes, Kapan's header at the back post threatened, Greg Leigh doing well to block it behind. From the corner, Lewis had to stretch to push the ball clear under pressure, falling awkwardly and needing extensive treatment before continuing.
Campbell was next into the book in the 67th minute, late on Colak who went down like a sack of potatoes. Logan was then flattened by a late tackle from Loncar in the 72nd minute and finally, a Rijeka man was given a yellow card.
James Wilson joined the fray in place of Hedges on 75 minutes, Muric being the first substitution for Rijeka in place of Loncar at the same time as the game gradually meandered towards its close, Rijeka happy to play keep-ball without really looking to penetrate while the Dons worked hard to keep them at bay.
Going into the last eight minutes, Rijeka made a double change with Iglesias and Vuk joining the by now rather tepid action in place of Acosty and Halilovic.
Having had himself a quiet 20 minutes, the referee popped up again to book Main for winning a header with two minutes to go. Seven yellow cards and a red produced on an evening with barely a bad tackle in it.
A night best forgotten, the Dons now need to bounce back from three straight defeats in a huge game at Dens Park in the Betfred Cup on Sunday.
"Even from the first minute, we had a free-kick and a chance to get the ball in the box, we played it short and got ourselves into a bit of bother.
"The first goal comes from our corner kick, one or two moved out of their slots and it allowed us to be counter-attacked.
"At 1-0 losing that first goal it was like a balloon popping,
"The fans did their job, they turned up and unfortunately we haven't done our jobs well enough.
"Rijeka are more than decent, I think its disrespectful to think that they are an average team, they are a good team and one that's well capable of taking advantage of our poor decision making.
"I can only analyse it up until the sending off because after that I think it's unfair to criticise anyone because the game is effectively done.
"An awful decision from the referee to deem that as two yellow cards, I have never seen the like of it. Other challenges were deemed not yellow card offenses which surprised me based on what he thought Funso had committed.
"Really disappointed with that and I need to mention it. It's poor decision making from us, awful decisions from the referee have just made it a very disappointing night.
"It's a situation you don't find yourself in very often - at 1-0 down, down to ten men against a very good side and when your fans have turned up expecting everyone to throw everything at it.
"From them on in I was thinking about the Dundee game, not getting players injured and keeping people fresh.
"In the second half we got the clean sheet that we asked for, people taking responsibility to at least do that but we have to take full responsibility for the way we started tonight.
"We needed everything to go our way tonight, the perfect performance. I thought before the game we had that big performance in us but it just unravelled and ran away from us as the night went on.
"I don't think Funso was naïve, he was caught on the ball and he put a challenge in. The referee has put the final nail in the coffin sending him off. I need to stress that I am not blaming the referee but he had a big part to play in that tonight.
"There are very few positives from it (Europe) other than the experience for some of the younger ones just coming into the club, of what is expected here.
"We now need to dust ourselves down, we are out of Europe and need to focus on domestic matters starting with Sunday.
"There will be loads of doom and gloom, people doubting my team, I believe we have put a squad together that will do well domestically, and we have the full season ahead of us to go and show that. Thankfully there were no more injuries tonight, Bryson has a chance to be fit for Sunday and we will be ready re-energised for the task of trying to get the club into Europe.
"I wasn't totally honest about Sam because he was fit, I wanted them to know for the first time that he was in the team when they saw his name on the team sheet. We waited and never named the team yesterday, although we did some structure and we did some work.
"Sam was keen to give it a go and he was fit but we took him off after 50 odd minutes because we didn't want to put him at risk ahead of Sunday.
"We need to analyse the performance tomorrow and shoulder the responsibility. We need to make sure we are ready for Sunday, it's a different challenge again but it's a game that's really important for us to give a response.
"I have watched Dundee a couple of times this season already, they have tried a few different systems and I think it will be a different system against us again. They have got plenty of experience in their team, a lot of new signings, they have a top flight front two and players who we are well aware of. I am pretty sure they will be confident that they can get a result against us but we need to get the job done."