City were at fault for each goal: Sylvain Distin failing to clear the first, Trabelsi giving the ball away for the second and Richard Dunne failing to clear the third, although Ronaldo was in an offside position when the cross came in.
"We came and had more chances on goal than probably any team will have here this season, but we shot ourselves in the foot by giving cheap goals away," said Pearce.
"We gave them a goal after 6 minutes, then on the verge of half-time I was thinking that on the run of play we would go in 1-0 down, so I am disappointed that the second goal went in at that point.
"I told them at half-time that if we got a goal at any time in the second half there would be nervous times at Old Trafford, and we got ourselves back in the game but then we did not really put pressure on them. I really thought we could have got something from the game, I really do.
"At 2-1 I wanted us to really roll our sleeves up and have a tonk at them, but we lost Micah Richards, who is a threat at set-pieces, and we were not able to generate the momentum we needed after the goal."
Pearce also backed Graham Poll's decision to give Bernardo Corradi a second booking when he dived late on.
"The referee got the decision right, simple as that," Pearce added. "I'll have a chat with Bernardo this week and tell him to keep his feet in the box and when someone does hit him, if it knocks him off his feet, that's fine.
"I'm honest enough to say that; I don't know how many other gaffers in the league would say the same."