Tevez has again declared he'd like to return to Boca Juniors.
"Tevez is like all great champions, indispensable for the great teams," Allegri said.
"Tevez is having an extraordinary season, and he must continue that, because we haven't reached any goals and still have two months to go.
"The last game will be [the Coppa Italia final] on June 7 so we have to work and try to achieve our objectives.
"At the moment the goal is to do well against Monaco, then we'll get back to League business."