Bebe moved to United in a surprise £7.4 million deal from Vitoria Guimaraes in 2010 but struggled to make an impact at Old Trafford and joined Turkish side Besiktas on a season-long loan in June 2011.
Bebe, who Sir Alex Ferguson later admitted was the first player he had signed without having even seen him play on television, made just a handful of appearances for Besiktas before suffering a serious knee injury on Portugal Under-21 duty that sidelined him for most of last season.
The Portuguese forward, now 22, insists he is fully fit and raring to go at Rio Ave, telling club website www.rioave-fc.pt: "I'm ready to play and pleased to wear the Rio Ave shirt.
"I'll give it everything while I'm here and try to help the team to continue occupying the places that they have been in the table."
He added: "Obviously something went wrong in the last phase of my career, I had a serious injury which thankfully I have fully recovered well from.
"I'm fine physically, I was always training and playing. It's good to come here and I want to show what I can do."
Rio Ave are fifth in the Portuguese top flight on 18 points, 14 behind leaders Benfica.