The Arsenal manager Arsene Wenger says Monaco were fortunate to reach the Champions League quarter-finals on away goals, after the English Premier League team's 2-0 away win was not enough to make up for their first-leg loss.
French division side Monaco won 3-1 in London three weeks ago but were overwhelmed by a dominant Arsenal side in the return although the visitors paid the price for wasting several good chances.
"We knew that it would be tough to score three goals but we did not seize our opportunities," Wenger says.
"It's not a question of merit, it's a question of being efficient. We also made psychological errors in the first leg. Monaco were fortunate, we must congratulate them."
However, Arsenal's Germany defender Pert Mertesacker was critical of his team and thought Monaco deserved to advance because of their performance in London.
"The best team went through. Monaco deserved it because they played much better in the first leg. We played well today but it was a massive deficit, and it wasn't enough tonight," Mertesacker says.
"We came here and tried absolutely everything, and when you look at the game we could have scored more than two. Monaco deserve it and that's absolutely fine."
Arsenal went ahead in the 36th minute when Olivier Giroud fired into the roof of the net and second-half substitute Aaron Ramsey doubled the tally 11 minutes from time with a low shot.
Atletico Madrid kept alive their dream of a second consecutive Champions League final appearance when they beat Bayer Leverkusen on penalties to claim a place in the last eight.
The Bundesliga side won last month's first leg in Germany 1-0 and a goal from Atletico midfielder Mario Suarez in the 27th minute of the return match left the teams locked at 1-1 after extra time at the Calderon.