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.
Manchester United continued their strong Premier League surge under interim manager Michael Carrick with a controlled 2-0 victory over Tottenham Hotspur at Old Trafford.
Manchester City boss Pep Guardiola has hailed Erling Haaland as the "best striker in the world" despite the Norwegian's dip in form, but refused to confirm if he will start Sunday's crucial clash with Liverpool.
Manchester United defender Harry Maguire has praised the growing influence of Jonny Evans and Jonathan Woodgate, insisting the coaching duo had already made a strong impact under interim head coach Michael Carrick.
Pep Guardiola is facing a major selection concern ahead of Manchester City's Premier League clash away to Liverpool on Sunday, with captain Bernardo Silva a doubt.
