Ernesto Valverde said Barcelona's failure to deal with Roma's "aggression" was the main reason why his side exited the Champions League on Tuesday night.
Barca travelled to Italy with a 4-1 lead from the first leg but goals from Edin Dzeko, Daniele De Rossi and Kostas Manolas helped Roma complete an improbable turnaround and advance to the semifinals on away goals.
The Serie A side, roared on by a Stadio Olimpico full of belief, took the game to Barca from the first minute and were ahead inside the first 10 minutes through Dzeko.
Valverde's side never recovered from that early setback and spent the majority of the game pinned back in their own half, unable to get Lionel Messi into the game.
De Rossi's second-half penalty and Kostas' late header sealed Roma's passage to the last four as the La Liga leaders left the competition at the quarterfinal stage for their third time in a row.
"The truth is that it was a game in which we couldn't play our football," Valverde said in his postgame news conference. "[Roma] were very aggressive, they played very well and we have to congratulate them.
"Everything went well for them. They scored early and got themselves right back in the tie. Then they scored [again] early in the second half and at no moment were we capable of getting into the game because of how well they played.
"They were aggressive and they took the game in the direction they wanted it to go. It was a bad day for us because couldn't overcome their pressure. You had the sensation that they could get the third goal at any time and in the end, they did."
Valverde, who named the same side as the first leg, didn't make his first substitution until the 81st minute and said the buck stops with him for the defeat.
"The coach is the one who has to take responsibility," he added. "[The coach] chooses the team, the changes, he plans the game, the season... so I am absolutely responsible for everything."
While Barca's Treble dream died in Rome, they can still win a Double this season -- they're 11 points clear at the top of La Liga and through to the Copa del Rey final.
"We still have important objectives ahead of us, but to win any trophy is difficult," Valverde said. "Now we will have some tough days ahead of us, I have no doubt about that.
"But we have to focus on the competitions we have left. We have to try and win them because we have not won anything yet. The defeat hurts, but now we need to look forward."
Barca president Josep Maria Bartomeu, meanwhile, admitted that Roma deserved to progress, apologising to the club's supporters in the process.
"Roma were better," he said. "I feel bad for the fans. Roma dominated the game, they believed in themselves. When you believe, when you throw everything forward and you give your all, these things can happen.
"I feel bad because we wanted to reach the semifinal, but we didn't deserve it today. The message is that we're sorry for all the Barca fans around the world. We have a good, competitive team, but we didn't see that today."