Isak has had a low-key introduction as a Liverpool player after his controversial £125m move from Newcastle.
Ahead of Liverpool's clash with Manchester United this weekend, Aldridge wrote in his column for the Liverpool Echo: "After over a month as a Liverpool player, it's time now for Alexander Isak to come to the party.
"It's fair to say there have been explanations rather than excuses as to why he is not up to 100% after arriving at the club under-cooked on the back of having no pre-season at Newcastle United.
"But having said that, the other side of the international break means he should be able to show exactly why he is the £125m man at Anfield.
"Isak still has the ability to bag 20 to 30 goals for Liverpool this season but the sooner he moves away from all the talk of needing to get fit, the sooner he will be able to start contributing properly for Arne Slot and do what he was signed for.
"During my playing days, people used to talk of needing a month or so to get up to match fitness over the summer and Isak will have had about six or seven weeks by the time Manchester United arrive at Anfield on Sunday afternoon."