Rashford has endured a difficult season since his belated return to action in October following shoulder surgery.
The England international was in United’s starting line-up for the midweek Champions League last 16 first leg draw at Atletico Madrid.
But, not for the first time this season, he failed to have an impact and was substituted with 15 minutes left.
Rashford’s replacement, Anthony Elanga, netted the equaliser and Rangnick made no attempt to hide his star’s struggles ahead of Saturday’s Premier League game against Watford.
“He’s definitely not injured, he definitely has no issues with his shoulder any more,” Rangnick told reporters on Friday.
“I think we’ve talked a lot about Marcus in the last couple of weeks and I have personally spoken a lot with him.
“It’s with him like with all the other players, it counts if they are on the pitch and they just have to perform.
“I’m pretty sure that he’s not happy with his performance right now but it doesn’t help if we discuss that every week.”
The 24-year-old has only five goals in 22 appearances this season.
But, challenging Rashford to look on the bright side after some good training performances recently, Rangnick added: “I don’t see any reason why he shouldn’t be happy.
“Our results have been good in the last couple of weeks and months, the team itself is in good shape.
“He’s right now trying to find his best possible shape and performance and we just try to give him a helping hand.
“In training he’s in top form, that’s why I decided to play him, but now it’s about having that transformation from training performance to the competition in the league, and this is the next step that he has to take.”