Roma coach Luciano Spalletti was at a loss to explain his side's lack of discipline as they crashed out of the Champions League on Tuesday.
After drawing 1-1 away to Porto, Roma crashed to a 3-0 defeat at home to suffer a 4-1 aggregate loss against the Portuguese outfit in their play-off.
Roma finished Tuesday's contest with nine men after Daniele De Rossi and Emerson were shown straight red cards, adding to Thomas Vermaelen's sending off in the first leg.
Spalletti was disappointed with his side's discipline but conceded there was "no warning" for that type of behaviour from his players.
"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," the 57-year-old 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 naive, 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.
"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."
He added: "We can list the 15-16 games from last season where we didn't get any red cards. 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 team-mates 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."