Roma manager Luciano Spalletti said he "can't understand why" his side picked up so many "naive" red cards as they crashed out of the Champions League on Tuesday.
Thomas Vermaelen was unavailable for the second leg against Porto after being sent off in Portugal, and Daniele De Rossi and Emerson Palmieri also saw red in the return leg as Roma fell 3-0 and 4-1 on aggregate.
Neither player could have any real complaint after clumsy tackles, and Spalletti was at a loss to explain them.
"What happened is we lost a very important match that will change the season going forward and we are going to have to deal with that," Spalletti told Mediaset Premium.
"We didn't control the ball well, which was what I had been planning and why I made certain choices. We didn't manage it, just as we were re-organising ourselves after the goal, we went down to 10 and it all became more difficult.
"The three red cards were naïve, because finishing the games with 11 men was our characteristic last season. We never gave the referee the chance to send us off. Now it's happening a lot and I can't understand why, as we could've been a lot calmer."
Roma only had five players sent off in Serie A last season, and none in the Champions League.
"We can list the 15-16 games from last season where we didn't get any red cards," the manager said. "This is new to me. And the first to get red cards were experienced players with quality, as Vermaelen was the first to go and he had been with us for three days.
"It's a new thing for me, but the team until this moment had never interpreted the game this way. We have our own rules and commandments, which includes no tolerance for those who leave their teammates down to 10 men.
"We are accustomed to behaving very differently. The last week represented a strange and different epilogue. We had no warning this was going to happen, so we couldn't prevent it."
De Rossi, nominally a midfielder, had been forced into covering at centre-back with Vermaelen suspended, and Spalletti said he could return there despite his red card.
"We reached the tie with quite a few injuries and I chose De Rossi as a centre-back because he could keep the ball better and has the qualities to play there," Spalletti said. "It may well be one of his future roles, for the way he interprets the midfield position.
"We lost the ball too often with silly errors, which were unforced, and that made Porto feel more comfortable to play their game. We got irritated, as we were unable to move the ball around well, then after the goal and red card it all fell apart.
"It's not possible in two legs of a play-off to be down to 10 men after 40 minutes."
Roma midfielder Kevin Strootman said there was nothing wrong with Roma physically.
"We had prepared for it well, but we started badly," Strootman said. "We played one game in 10 and the other in nine. It could be a mental problem because physically we are fine. We knew what we had to do on the pitch, but we didn't do it. We didn't play well and I don't know why.
"Even with nine men we tried everything. But when we went 2-0 down it was all over. Now it's difficult to get back up, we are all angry. But we have to think about the league because we have to do better. No one is to blame, we lost together."
Conversly, Porto coach Nuno Espirito Santo hailed his side for having the proper mental approach.
"We stepped on to the field with the right approach and mentality," said Espirito Santo. "We did well to control the game against a great team, also playing good football. We were able to neutralise Roma, a great team from a big club.
"We were concentrated and I have to publicly congratulate my players, as they did a fantastic job against a quality opponent.
"I could see they were helping each other out and truly believed. It was important to defend with discretion and play in a relaxed fashion.
"We deserved this qualification, cancelling out Roma's strong points and ensuring they never felt at ease. This is the Porto we want."