Mancini was pleased with Zenit's display in Saint Petersburg as they sealed a 3-0 win on the night and a 3-1 aggregate victory, and blamed the first-leg defeat on not having played for two months given the Russian Premier League's lengthy winter break.
"We played better than Celtic," the former Manchester City boss told reporters. "I think we played good game in Glasgow, but it was the first game after the winter break.
"The games were different for this reason. We were so tired after 70 minutes, but that is normal.
"I don't feel that we can win because there are many strong teams and we are a young team."
Zenit return to action on March 3 with a league home game against Amkar Perm.