Man United lost 1-0 at St James' Park on Saturday as Rashford was substituted after just 61 minutes.
The England forward scored 30 goals in all competitions last season but has managed just two in 18 games this term.
"I know this issue is coming up and Marcus is investing a lot, we support him and he will return to his form," said Ten Hag.
"He works hard and he's investing a lot. He will get back, he will recover and he gets all our support."
United have now lost six Premier League games this season and 10 in all competitions. They managed just one shot on target against Newcastle while Eddie Howe's side recorded 22 attempts, including Kieran Trippier's free-kick that hit the crossbar.
"Today we have to say the credit's to Newcastle," said Ten Hag.
"After the start, where we could have scored with Alejandro Garnacho, they were better than us.
"They were more proactive and we had to go behind to go back. We allowed them one goal and in the end we fought back. We had two good opportunities but we couldn't take a point from there.
"I will talk with my team about the reasons but we have to do better and, as I said, I am sure they will pick this up, I am sure they are resilient and they have the character to do this."