Rondon has been sidelined since limping out of last month’s goalless draw against Crystal Palace.
The striker, signed on loan from West Bromwich Albion, is not available for this afternoon’s home game against Brighton and Hove Albion.
United manager Benitez said: “It hasn’t really been too frustrating, because the first (injury) was when he was at West Brom and we couldn’t control that, because he wasn’t our player. He came here, and then he needed to work a little bit on his fitness.
“The second one was against Crystal Palace, when I think he wanted to impress.
“He said ‘I’m fine’, but you could see in the first half that he wasn’t right. Then we made a substitution. He wanted to continue, but we could see he was not ready.
“Sometimes players, when they want to do well, they take the risk – and he took the risk.”
Rondon could return against Southampton next weekend.
Benitez said: “ Now he’s OK, and he has been training this week. We have been pushing him with the fitness coach, but he’s been training with the team.
“He’s feeling better, and I think he’s in a good mood now. Hopefully, if he’s fine, next week we can see him with the team.”
Meanwhile, Paul Dummett will return against Brighton, but Ciaran Clark remains sidelined.
“Clark’s had some treatment for a muscle problem,” said Benitez. “Dummett has been training normally.”