Isak has just returned from a double leg fracture suffered before Christmas.
Rush told talkSPORT: "Well, I think he's going to be more ready for next season.
"I think now, because as you know, once you miss pre-season, you're playing catch-up all the time.
"So I'm glad he is back, because he's probably our proper number nine; we haven't had a proper number nine.
"Isak is a number nine, and I think maybe we've missed a number nine."
On Liverpool's season, Rush also said: "It's been indifferent, I think last year we surprised teams, we won the league, we were surprised we won the league, but I think this year people want to beat us cos we won the league, so I think with injuries as well, it has been indifferent, but there's still a lot to play for."