Rashford limped off late in United's 2-1 Europa League playoff second leg win on Thursday (February 23), casting doubt over whether he will start against Newcastle United on Sunday, as his team look to win their first trophy since 2017 at Wembley.
No player in Europe's top-five leagues has more goals than Rashford since the World Cup break, with his 24 strikes this term the most he has ever scored in a single campaign in his career.
"I don't know, the players now are coming in, we have to do investigations, medical of course we did straight after the game, but most of the time you have to wait for a 100 percent diagnosis," Ten Hag told a news conference on Friday.
"So let's do the medical, the work, we have to wait."
Thursday's victory kept United competing in four competitions this season as they continue to go from strength to strength under Ten Hag.
Ten Hag's team will be looking to defeat Newcastle United for the side's first silverware in six years.
"The fans are really waiting for it. You feel it. So we will do everything we can to give the fans that honour," said Ten Hag.
"(Newcastle) is a great team, a clear philosophy about how they want to play the game. They're an annoying team to play against, so we have to find a way to win. They try to annoy you. We have to play our game and focus on our game."
Ten Hag also confirmed French forward Anthony Martial remains out injured and will not take part on Sunday.