United are reported to have agreed an £18.9m fee for the keeper and Sir Alex Ferguson hopes to wrap up a deal soon.
Former United forward Forlan told BBC Sport: "David will become a top player because he has a lot of ability.
"But he's only just had his first full season as Atletico's number one and is a young guy with little experience."
De Gea is currently playing for Spain at the European Under-21 Championships in Denmark, and says he wants to wait until the end of the tournament before signing a deal with United.
"There's no doubt that David will become a very, very good player, because he has the talent," Forlan said. "He is tall, strong on crosses, a good shot stopper and distributes the ball very well.
"I suppose he is similar to (Edwin) Van der Sar in a lot of ways. Even though he is very young, he gives a lot of confidence to his defenders and has a lot of talent."
However, he does have concerns that De Gea, who came through the youth ranks at Atletico, could become homesick in Manchester.
"You never know for sure whether a player will adapt to a new country until he gets there," the Uruguayan said. "It's true that I was also young when I joined United, just 22, but you can't compare our situations.
"I knew the language and had already left my country, having moved to Independiente when I was only 19. So it was not such a big step for me to go to United.
"David is from a little town outside Madrid and has been at Atletico since he was very young. He doesn't speak any languages other than Spanish. He is going over with his girlfriend and family though, which will help."
Forlan has also advising telling de Gea on what life is like in Manchester.
"I told him that Manchester United is a great place to experience as a player and that I had a great time there," he added.
"The manager and staff are fantastic and the fans were always very supportive of me, even when I wasn't scoring. I made some good friends there and settled in well. Maybe the weather wasn't always what I wanted, but I had a great experience.
"David really wants to go to United, there's no question about that. He wants the challenge and he wants the experience. United is one of the biggest clubs in the world and not many players get the chance to play there."