Isak took no part in preseason and hasn't been considered for selection by Newcastle as he pushes to join Liverpool.
But Tomasson included Isak in his squad for next month's World Cup qualifiers against Slovenia (September 5) and Kosovo (September 8).
"Can he play 90 minutes? Of course not. Not at all," explained Tomasson.
"Alex is a very professional player, a strong character, and he has worked hard. He is a game changer. I have spoken to him several times; mentally, he is strong. What we have said stays between us, of course."
The Dane added: "It is very difficult to have a clear opinion about the situation. We do not know what has happened in the club. We must be very careful about drawing hasty conclusions when we do not have the whole picture. He is smart enough, he is smarter than me I think."