The comments came after the forward made his first appearance for the club in 20 months during Saturday's 4-2 pre-season defeat by AC Milan.
Rashford came off the bench in Wroclaw and produced a moment of skill by the corner flag before going close to scoring after being handed the number nine shirt by manager Michael Carrick.
Manchester United have stressed the number is not being treated as permanent, with intended first-choice striker Benjamin Sesko missing through injury, but the gesture underlined the uncertainty still hanging over Rashford's future ahead of the September 1 transfer deadline. Carrick admitted last week he could not say whether the England forward will remain at Old Trafford, and Maguire said he is equally unsure, though his preference is clear.
"As players, we hope he stays because if he does, he is a big asset to have. He can improve us for sure," Maguire said. "You saw when he came on, he threatened the backline really well.
"He could have scored. He's a huge threat. I've played with Marcus for numerous years and I know that playing against him and playing with him in training is going to help us."
Maguire opened the scoring inside two minutes with a header from Bruno Fernandes's corner, but AC Milan hit back through Samuel Chukwueze before goals in the closing stages turned a 2-1 lead into a 4-2 defeat, with Bruno Fernandes also missing a penalty for the second match running. Maguire accepted it was United's worst performance of a long pre-season, coming just a week before the Premier League opener at Hull City.
"We're wanting to play much further up the pitch, be more aggressive and more on the front foot," Maguire said. "This was the only game where I didn't feel we did ourselves justice in that aspect. We had a lot of high regains and created a lot of chances from that, but we didn't sustain our pressure anywhere close to enough."
Maguire captained the side in several of the earlier pre-season matches when Fernandes was absent, a possible indicator he could take on the vice-captaincy this season, though that is yet to be confirmed. There was also a warm reunion with former manager Ruben Amorim, who returned to English opposition for the first time since his sacking by United in January, with Rashford and Kobbie Mainoo, both of whom fell out with the Portuguese during his spell in charge, seen embracing him and his coaching staff after the match.
Speaking to Gazzetta dello Sport, Amorim said he remains fond of his old club.
"I'm proud to have been at Manchester United and I still love Manchester United," he said, joking with journalists he recognised from his time in England that press conferences "are more boring" without him.
Maguire, too, had nothing but praise for his former manager.
"It was great to see him and all his staff," he said. "I got on with Ruben really well and all his staff. I am happy he has another chance to impress and show everyone what a great manager he is. I'm sure he's going to be a success. I have nothing bad to say about Ruben. It just didn't work out at Manchester United. But he's got a massive club now. I'll be following them and hope they have an amazing season."