The Argentine ace, who has netted 15 times in the league this season, will miss Monday night's clash between the two sides because of a hamstring injury but Mourinho is of the belief that there are plenty of capable strikers in Manuel Pellegrini's squad who can do the job.
"He's a fantastic player, but he's one of so many," Mourinho said.
"Even the fourth striker (Stevan Jovetic), coming off the bench, comes on and scores (in Wednesday's 5-1 win at Tottenham).
"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."
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