Muto, signed from Mainz in the summer in a £9.5million deal, has played 65 minutes of league football so far this season.
The forward came off the bench late in Saturday's goalless draw against Crystal Palace.
United manager Benitez feels that the Japan international needs more time to adjust to the physicality and intensity of English football.
Asked what Muto needed to do to get a league start, Benitez said: "What I can see every training session is commitment, passion, workrate, but still you can see he was fouled a couple of times (against Palace) and the referee didn't give a foul, so he was complaining and waiting.
"So he needs to understand that the Premier League is different, and then you have to carry on.
"Some players, they need one year. Some players, they six months. Some players, they need one month.
"So, for a new player, always you have given him some time, and he has to be as positive as he is in every training session."
Muto's only start so far came in last month's Carabao Cup defeat to Nottingham Forest at the City Ground.
The 26-year-old signed a four-year deal at St James's Park.
Muto revealed last month why he turned down a move to Chelsea three years ago.