Rashford's winner in Thursday's Europa League last-16 second leg victory over Real Betis was his 27th for United this season while the 25-year-old also scored three times for England at the World Cup in Qatar.
After scoring only five times last term, Rashford has been top scorer in the League Cup -- where he netted in every round -- as well as in the Europa League, with six strikes in each competition.
"I said it in the summer, I was asked if I believe in Rashford or (Anthony) Martial, (if) one of them could score 20-plus goals and I said: 'Yes, I believe,'" Ten Hag told reporters ahead of Sunday's FA Cup quarter-final at home to Fulham.
"He (Rashford) progressed during the season. He was not in the best shape when he started the season but the way of playing gives him a base and he brings his skills in and his attitude and mentality.
"That is bringing in a lot of progress and gives the team a lot of joy and gives him a lot of goals and us a lot of wins ... I knew his skills. You also bring staff around who can make him better and make him progress and he can get motivated from."
Ten Hag said winning the League Cup should act as fuel in United's quest for the FA Cup as he looks to restore the mentality of a "winning culture" that made the club successful in the past.
They will be without close-season signing Casemiro, however, after the Brazilian received the second red card of his United career last weekend.
"We did really well without Casemiro against Arsenal, Leeds (United) and also at the start of the season, but I want to have Casemiro available. He has a huge impact on our game," the Dutch manager said.
Ten Hag added that he had no regrets in letting midfielder Andreas Pereira move to Fulham where the midfielder has three goals and six assists this season.
"He's a good player and I know him a long time ... I'm really happy for him and I think it was the right step, he had to play on a regular basis," Ten Hag said.
