Crespo was originally sent to the stands, but Zlatan Ibrahimovic's thigh twinge meant he was called at the last minute and eventually came on as a substitute to score the equaliser.
"It's true, if Ibra was fit then Crespo would not have been on the bench, as his place was taken by Balotelli," said Mourinho.
"I too am very pleased for him, because he is a fantastic guy, a player who experiences this moment without knowing his future and giving his all for Inter. I am extremely happy for him.
"He works hard, but the problem is that everyone wants to play. It's true he came on and made a real difference. This was his type of game in those final stages, as Roma were 3-2 up and our crosses were not connecting enough in the box.
"I could've put [Amantino] Mancini on the left and Mario in the middle with Adriano, but I saw Balotelli was doing well down the flank.
"Plus Crespo has a real nose for goal and got his first of the season. It is a point, but above all a reward to the team for the intensity of the fightback."