"The way he's performed to the end of the season has been incredible, really, for his age," Rooney told a news conference after a training session on Wednesday.
"He showed what he can do in his first game for England," added the 30-year-old, who made his England debut at 17 and will lead the side against Portugal in their final Euro 2016 warm-up at Wembley on Thursday.
"His attitude is first class ... he could be a real positive for us."
Rashford, 18, became the youngest player to score on his England debut in the 2-1 win over Australia at the Stadium of Light last Friday.
The forward scored that goal 138 seconds into what was only his 19th professional match.
England manager Roy Hodgson said that in the end it was a question of whether to take an extra attacker or midfielder in his 23-man squad, rather than the "new boy on the block" having to make his case.
"It certainly was never going to be a case of 'We'll put Rashford in for this game and if he does well, we'll take him. If he does badly, we won't'," he said. "It's always been the balance in the squad ... rather than X v Y who play in the same position."
Hodgson said the mix in his young squad was good.
"If anything, it's tilted towards an offensive approach but that's because we believe we have so many players at the moment in midfield and in attack who are really good-quality players," he said.
"I didn't want to sacrifice them to shore up with players who maybe might in some ways cover positions slightly better but maybe don't have the qualities of the players I'd be leaving out," he added.
"We believe that defending is very much a team job ... we think it starts from the front and goes right the way through."
Hodgson said England needed mobility and great running power to be able to do that and his squad, with its younger players in particular, could offer that.
"I believe that if they perform anything like to the level they are capable of performing, we are going to be a hard team to beat," he said.
(Reporting by Alan Baldwin, editing by Neville Dalton)
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