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."
Last month, Ferguson said he regarded De Gea as the natural replacement for Edwin van der Sar, who retired after United's Champions League final defeat by Barcelona, and admitted he had been tracking the Spaniard for "quite a while".
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.
Forlan, 32, who played for United from 2002 to 2004, has been hugely impressed with De Gea since the Spaniard forced his way into Atletico's team towards the end of the 2009/10 season.
"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 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."
Forlan joined United for £7.5m from Argentine side Independiente, but failed to cement a regular starting place at the club. He famously went 27 games before scoring for his new side and only hit the net 10 times in 63 games before moving to Villarreal.
David is from a little town outside Madrid, has been at Atletico since he was very young and doesn't speak any languages other than Spanish
He admitted 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 revealed that De Gea had been hungry for insights into life with the English champions.
"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."

Daniel James' late penalty gave Wales a first win in the competition after an opening draw against hosts France.
Page's side face Ivory Coast in their final Group B game on Monday.
"It's always a good sign when you win games of football and you haven't been at your best," said former Wales defender Page.
"We probably played better against France and didn't win.
"If someone would have said before the tournament that we'd be two games in and with four points going into the third game I'd snap their hand off.
"It's our first time in this tournament and we've been up against excellent opposition against France and a different challenge against Bahrain."
Wales are second after their win over Bahrain with next opponents Ivory Coast top of Group B after they beat France.
Diego Forlan expects his Atletico Madrid team-mate David de Gea to be a big hit at Manchester United - if he can adapt to life in England.
[NEXT_CONCEPT]
Intermediate teams manager Rob Page says Wales Under-20s were under par despite a 1-0 win over Bahrain in the Toulon Tournament.