Hargreaves is available on a free transfer after being released by United this summer. The midfielder had a medical at rivals Manchester City on Tuesday.
“He worked really hard and got back last season, played one game and then got a hamstring injury,” Rooney said.
“He’s been out for a couple of years. I think it will be difficult for him to get back to his best. I hope he can do so.
“From an England point of view he was a type of midfield player that we hadn’t really had before.
“If he can get back that would be great but it will be difficult for him.”
Hargreaves was released by United after spending four years with injury problems at Old Trafford.
