The 18-year-old completed his move to United from Atalanta, which was originally agreed in October, this month.
Diallo did not start a competitive game for Atalanta, only making a handful of substitute appearances.
But Solskjaer has been impressed by how the winger has adapted so far and suggested he could be on the pitch for the first team within a month.
"Very good [first impression]," Solskjaer said about Diallo.
"He can handle the ball, he has enjoyed training, makes a difference in training, which is quite remarkable at his age.
"Some adjustment to settle into the hustle and bustle of the Premier League but I can't see it's too long until he’s in the squad."