The performance helped secure a valuable three points for Ole Gunnar Solskjaer's side, moving them up to sixth on the table.
Rashford now has 12 goals from his latest 13 games, and De Gea is delighted with his form.
The Spanish goalkeeper told the club's official website: "Marcus is in very good form and showing his talent.
"He is a great finisher. He scored two and could have scored even more goals.
"We are happy that he is scoring goals for us and playing as he is doing. He is a great player."