The Argentinian has been in impressive form for City, having scored 26 goals in 25 appearances in all competitions this season.
Mourinho told reporters: "He's a fantastic player, but he's one of so many. Even the fourth striker (Stevan Jovetic) comes on and scores [at Tottenham Hotspur].
"For the last months Sergio was injured and the third striker, [Edin] Dzeko, was making the difference and scoring a lot of goals. So for them it's not important if somebody is injured."
The two teams face each other at the Etihad Stadium on Monday.