Rashford returned to Carrington in the summer after spending 18 months on loan between Aston Villa and Barcelona.
The 28-year-old bagged 14 goals and 14 assists for the Spanish giants, but the club decided not to trigger his £26M release clause to sign him permanently over the summer.
Instead, Barcelona signed Newcastle's Anthony Gordon for around £69M and he has since thrived at the side, grabbing 7 assists and 1 goal in just 9 appearances.
Speaking to ComeOn, Gullit revealed that Rashford needs to up his game at United and prove himself after a start which sees him with 0 goal contributions after 5 games.
"Of course, it didn’t happen for Rashford, but he was only there on loan, and I think he learned a lot playing for Barcelona. He played with very good players around him there and now he has to do it at a different team, playing different football, without those players, and I don’t know if he likes that. I don’t know if he wants it anymore.
"We will see because he will show us and I know he wants to play well but it’s not nice to leave the love of your life, United, and find out that a lot of people didn’t even want you. That’s not nice at all. Now he has to go back and prove himself again and that is hard.
"Rashford wasn’t the big story around Barcelona this summer. Gordon was the big story, and I think it was a fantastic story because he visualised it. He wanted it. He went and studied Spanish and prepared for it. Now look where he is. That is a fantastic story, and I love it.
"It goes to show other players, if you can visualise it, you can make it happen too."
United's next clash comes against Tottenham Hotspur at Old Trafford after the international break. Spurs are bottom of the table and if Rashford is to get back to his old self this game may be the perfect opportunity.