Rashford notched the only goal of the game at Old Trafford to take Manchester United above Chelsea into fifth position on the table.
After a difficult period last season during which Rashford scored just five goals in 32 appearances, the England international has been reborn under Erik Ten Hag this term, with his latest strike taking him to seven goals in 15 appearances so far this season.
Rashford's winner against the Hammers made him the 22nd player to score 100 goals for Manchester United and the first since Wayne Rooney in 2009.
"He's still very young. I think he has been here his whole life as a kid and as a first-team player with experience already, and he's still very young," De Gea told MUTV about the 25-year-old.
"He's doing great. I think he can be one of the best players in the world if he keeps focused and training hard and performing well.
"I think he's a very, very good player and we are happy that he's with us and playing the way he's playing."
De Gea also gave credit to the Man United side, with this latest shutout their fifth in their last six games in all competitions.
"I think the whole team [are contributing], not just the defenders," he said.
"I think we are pressing well, we are recovering the ball high in the opponents half so that's massive for us. I think we defended really well.
"Of course, we conceded some chances at the end, but we're playing against West Ham, a great team who have good players. I'm really, really happy, to be honest."
