Mancini had said ahead of City's 1-0 victory over Tottenham that Tevez would have to play some part in the game if he was to feature at Wembley.
The nervy nature of the victory - which ensures City will finish in the top four - seemingly made Mancini reluctant to introduce Tevez at Eastlands, though, and the club captain appeared frustrated when told he needed to continue his warm-up in the second half.
He eventually came on in the 83rd minute and, although he looked sharp, Mancini has told the forward that his lack of match practice will hamper his chances of selection at the weekend.
"We have four days," Mancini said. "I want to take all the time to decide this. I don't think he can be 100% ready because he only played for 15 minutes and has only had one week with the team, but we have four days, then we need to decide."
For Mancini, the FA Cup final can now be viewed as a bonus rather than a requirement after he fulfilled chairman Khaldoon al-Mubarak's minimum target of Champions League qualification.
"When you have a target and get it, it is very important for your job, but I am happy for the supporters and the club first of all," Mancini said.
"We deserve to be in the Champions League because we were between first and fourth position all season.
"If we didn't throw three points out of the window against Everton on Saturday, we could probably finish third. I don't know how we lost this game. Now it is difficult [to finish third].
"We have two games left, but now it is very important to think about the FA Cup final."