The Argentine ace is still being heavily linked with an Eastlands departure this summer but Johnson has urged him, the best player he has played with, to stick around.
"He carries us sometimes with his goals and his drive," said Johnson. "He is probably the best player I have played with.
"Some of the goals and some of the stuff he does in training is just unbelievable. Personally I hope we keep hold of him just like all the City fans do, because he will be a major part next season when we are playing in the Champions League.
"He would be hard to replace - there's probably only five or six strikers out there who get you that many goals a season at the highest level.
"We have one of them, so it would be great if we could keep him."