The England international has only managed one goal in eight appearances at club level this season, in addition to providing two assists.
The 25-year-old netted against Arsenal in the Premier League at the start of September but has not registered since, and he again found it difficult to make his mark against Crystal Palace on Saturday.
Rashford was replaced in the latter stages of what proved to be a 1-0 loss to the Eagles, and there has been criticism of his overall performance levels early this term.
The attacker was the club's standout player last season, though, scoring 30 goals and registering 11 assists in 56 appearances in all competitions.
Ten Hag has said that if Rashford "works hard and invests every day" then he will overcome a difficult spell to become a regular goalscorer once again.
"The facts are he's not scoring at this moment, but he's had opportunities, like against Brighton, where he had five or six occasions in a very good spot," Ten Hag told reporters on Monday during his press conference to preview the Champions League clash with Galatasaray.
"If he works hard and invests every day, with the right focus and if the team around him supplies and helps him and supports him with movement, then it will come and Marcus Rashford is a player who's capable of scoring a goal in every game. When he's coming in the right spots, he will score."
Ten Hag was also asked about other underperforming players in his squad, with a number of the first team currently finding it difficult to show consistency this term.
"You have to deal with. Experienced players know how to deal with it and it's not the first time we're in this situation because that's a career, it's not only highlights, it's a journey and that will have bumps.
"Now it's about the character, what you have to show and how to get out. There are many positives but things we need to improve on."
Ten Hag also said that his side will need to be 'at their best' in order to claim a positive result against Turkish giants Galatasaray.
"We know that [Galatasaray quality], it's not only [Wilfried] Zaha and [Mauro] Icardi, but there's [Dries] Mertens and [Hakim] Ziyech, so they have a lot more and it's about the team. We have to defend that and have to be at our best to get the right result," he added.
Man United have made their worst start to a league season for 34 years, while they have lost five of their nine matches in all competitions this term.