The former Atalanta winger joined United last week and is now training with Solskjaer's senior squad.
Ahead of Sunday's 0-0 Premier League draw at Anfield, the United manager revealed Diallo could see action in the fourth round of the Cup.
“He's an exciting player who will get time," Solskjaer told Dagbladet.
“He's a young player who has really exciting qualities but he needs time to get used to the physical style in England. That speaks for itself when he's 18.
"We have a fantastic coaching staff which will help him to build a physicality which will suit the Premier League. But he's got all the ingredients to become a very good player, so we'll give him time.