The England international has struggled for form all season, scoring just two goals in 19 games, and Ten Hag said he has to earn the right to return to the team while highlighting the way Harry Maguire and Scott McTominay have battled back from the fringes of the squad to play key roles.
"Rashford is an incredible, good player," Ten Hag told a news conference ahead of their clash against Bournemouth.
"You can't do it with 11 players. He can't play every game and he's not, in this moment, in the form he was last year but I am sure he will get there.
"What I said about Scott McTominay and Harry Maguire, it's up to them. The team will always line themselves up and the best players who form the best team will play."
Maguire and McTominay were both linked with moves away from Old Trafford in the summer but have since become vital to United's revival under Ten Hag.
McTominay scored both goals against Chelsea and is top scorer this season with six, while Maguire was named the Premier League's player of the month for November.
"I think we have a squad and in the squad there is internal competition," Ten Hag added.
"If you want a successful season you need more than 11 players. In every season, it will turn and in every season it's what is the best team. They are here, they are great players and played into the team. That's what you expect from every player, there has to be dedication to aim for this."
"As manager, my coaches, with the rest of the staff, we always consider [things] but the next game is always the most important," he said.
"Especially in the Premier League where everyone is killing everyone, you need a strong team.
"I think they have very good form, Bournemouth. We have seen them, it's a very good side, very well constructed, capable players. You have to play your best."