The England international is thought to be suffering from a stomach bug, with interim manager Ryan Giggs telling him to stay away from Carrington to avoid passing anything on to his team-mates.
However, several newspapers believe the problem could be more serious, explaining why the club's medical staff is so keen to investigate further.
The Daily Mail reports that United will wait for results on Friday before making a decision on Rooney's fitness for the weekend's penultimate home game of the season against Sunderland.
Although with little left for United to play for this season, it is unlikely that Rooney will be risked. Certainly, that precautionary view will be endorsed by Roy Hodgson, with the England manager set to name his 23-man World Cup squad on May 12.
Rooney has entered his last two World Cups under an injury cloud and will be keen to avoid doing the same this summer in Brazil. In 2006, he was rushed in his recovery from a broken metatarsal before the tournament in Germany while in 2010 he had damaged ligaments in his ankle before heading to South Africa.
This latest scare comes in another injury-blighted campaign for Rooney, who has only just got over a chipped bone in his toe he picked up in the first leg of the Champions League quarter-final against Bayern Munich that required pain-killing injections before the second leg.
United face Sunderland at Old Trafford on Saturday before playing their final home fixture of the season against Hull City on Tuesday. They wrap up their campaign the following Sunday at Southampton.
- Sports & Recreation
- Ryan Giggs