The Liverpool signing did not take part in former club Newcastle United's preseason before Monday's move to Anfield.
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Tomasson called up Isak for this week's World Cup qualifiers, though left him on the bench for Friday's 2-2 draw with Slovenia.
"He had only done three training sessions with the team,” Tomasson explained to Viaplay after the game.
“No pre-season with the team and of course no playing time. The risk is probably a bit too great to use him today.”
Sweden meet Kosovo on Monday and Tomasson says Isak could feature from the bench.
“Hopefully, hopefully. As a game-changer,” added the Dane.