The Swede could make a return from a leg fracture before the end of the season.
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Rush told the Liverpool Echo: "You don't see proper No.9s now, do you? You've got (Erling) Haaland, (Harry) Kane and you've got Isak. I can't wait for him to get back because he's in my sort of position.
"I like to see a proper No.9 and hopefully we'll see him soon."
On choosing between Isak and Hugo Ekitike, Rush added: "With Isak it was a bit difficult because he missed pre season. He's a No.9.
"All you have to do to the manager is show them what you can do do on the pitch and they can't leave you out. If they're both performing, then they can both start in the same side."