The Nigeria international moved to Ewood Park in August from Everton.
Rathbone, who played more than 300 appearances for Rovers between 1979 and 1987, told the Lancashire Telegraph: "I think he is in the shape of his life. He's a big strong unit but he's in great shape at the moment. Don't expect him to run 30km in a game, but get the ball in to him around the box and he will score goals.
"Do you come back 100 per cent after an injury like that? That's debatable. But you can come back, look at David Beckham and countless other examples.
"It's credit to him, you have to put a lot of work in and he did that. It is one of the worst injuries you can get, I think it's worse than a cruciate ligament.
"I think he's looking okay at the moment. I saw him play against Newcastle and Arsenal. He just needs a run of 10 games and he will score goals in the Premier League, I've no doubt about that."