The 22-year-old has played seven times for the Red Devils and has found the net twice since joining the club August 2010 for a fee in the region of £7 million.
However, he has struggled to hold down a regular place in Sir Alex Ferguson's first-team due to the stiff competition amongst the United attack which includes Robin van Persie and Wayne Rooney.
The Scot feels the loan move will give Bebe a chance to play regular football for something which he can't get at Old Trafford and believes the move will give him the opportunity to progress as a player.
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He told the club's official website: “We feel the boy has got to get good experience at a better level than our reserve team.
“He’s gone to Rio Ave who are fifth in the Portuguese league at the moment – they have a young coach [Nuno] who has a very good reputation.
“Bebe has done quite well in the last two or three months. He got a cruciate knee injury when he was on loan in Turkey last season and that derailed him for a bit. Coming back from these injuries takes time but he’s started to develop quite well.
"Hopefully his spell in Portugal, where he is from anyway, could help him.”