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.
Virgil van Dijk has insisted Liverpool must "fight" and "earn" their place in the next round of the UEFA Champions League after slipping to a narrow defeat against Galatasaray in the first leg of their last-16 tie.
Manchester United defender Harry Maguire has praised former team-mate Victor Lindelof ahead of the Swede's return to Old Trafford with Aston Villa this weekend.
Manchester United manager Michael Carrick revealed a positive camp ahead of what he feels is an 'important' Premier League fixture against Aston Villa on Sunday.
Liverpool endured a frustrating night in Istanbul as they were beaten 1-0 by Galatasaray in the first leg of their UEFA Champions League Round of 16 tie.
