Metro reports that United have been told the Spain Under-21 player is available for a bargain £6million.
United boss David Moyes is desperate to bring in an attacking midfielder in January after having failed to land Cesc Fabregas and Ander Herrera in the summer.
Roberto has just 18 months left on his current contract with Barcelona and his agent has offered him around.
The highly-rated midfielder reportedly interested in a January move to Old Trafford.
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Liverpool have unveiled their new home kit for next season, sticking with the hugely popular retro styling of the current shirt.
The reputation of the Premier League means it is the most popular and one of the most watched leagues in the world.