Isak, 26, missed much of the pre-season due to transfer negotiations and had spent months sidelined at Newcastle. Since his Deadline Day move, he has featured in six matches - playing just 313 minutes and scoring once.
"He's now had his five or six weeks of preseason, which is normal after being out for a few months," Slot said ahead of Liverpool's clash with Manchester United. "Fitness-wise, he's close to the level he should be, and we can judge him fairly from now on."
Slot acknowledged Isak's goalless international break with Sweden but remained confident in the striker's development, noting that recent appearances of 70–90 minutes have helped him reach match sharpness.
Meanwhile, the Reds boss confirmed Ryan Gravenberch is fit despite a minor hamstring issue, and Ibrahima Konate has resumed training following a quad injury. However, goalkeeper Alisson remains sidelined and is expected to miss both the weekend fixture and next week's Champions League tie in Frankfurt.
Crystal Palace captain Marc Guehi has informed the club that he will not extend his contract and plans to leave next summer, manager Oliver Glasner confirmed on Friday.
Arsenal manager Mikel Arteta has given a worrying injury update on Martin Odegaard, admitting he is not sure how long the Gunners captain will be out for - only that it will be for "weeks".
England goalkeeper Jordan Pickford has signed a new four-year contract at Everton until the end of June, 2029.
Liverpool's preparations for their showdown with Manchester United have been thrown into disarray as Arne Slot faces a growing injury list just days before the Premier League champions return to action at Anfield on Sunday.
