The Sporting CP coach will move to United next week.
And Fonte, now with Casa Pia, told BBC Sport: "I think it was only a matter of time before he moved to a bigger club. What I like about him is that he has his own philosophy, he has his own ideas. The other day I spoke to one of his former players, Luis Neto, and he told me that he is one of those coaches who pays attention to detail, and sometimes it can be a bit too much, but that's what makes the difference in my opinion."
He continued: "I think he has the ability to be a great success at Manchester United. I know him personally, I've played against Sporting Lisbon a couple of times now, and I know how tough it is to play against one of their teams. The way Sporting play is incredible. They have threats coming from the back, threats between the lines, they build, they press and they know what to do in every game situation."
Fonte also revealed: "My brother was coached by him. I spoke to my brother about Ruben many times, even before he was linked to Manchester United and what he told me is that he was obsessed with football, with training and that is a very important thing if you want to be one of the best. In every job he has had, he has been so successful and that tells you something. He has something special and I think he will be successful because he is a hard worker."