The Montenegro forward joined City from Fiorentina for £22million last summer but was hampered by illness and injury during a disappointing first campaign with the club.
However, he has looked sharp in pre-season, making it four goals in two games in the International Champions Cup when he scored twice in Wednesday’s loss to Liverpool, which finished 2-2 before the Reds won on penalties.
Pellegrini said: "I have said before that Stevan is a very important player for us, and he proved again what an asset he can be.
"He had a number of different injuries last season but now he is fully fit.
"I'm sure if he continues in this way he will show why he is here. If he's fit all the year, he will be a very important player for us."
Despite the shoot-out loss, Pellegrini was satisfied with his team’s overall performance.
"I'm very happy because it was a good, competitive game and useful for both teams," he said.
"We played well for the first 45 minutes, when 0-0 didn’t reflect exactly what happened.
"We are preparing for the Community Shield and the Premier League, and that is the most important thing.
"Overall, I am very happy with our preparations. The five players who arrived on Thursday will be ready for the Premier League and the rest of the players will arrive next Monday.
"They can be fit in maybe 15 days but we will assess things as we go along."