The 30 year-old Aussie stand-off saw specialist Jon Webb in Bristol this week, and a MRI scan revealed bruising of the bone around the knee area.
McRae came off in the second half of Gloucester's game against Harlequins at the weekend, after four games back following a knee operation.
The former South Wales Waratahs half-back started the season in superb form, controlling the game from number 10, as Gloucester one six of their opening seven Premiership games.
Since the Heineken Cup clash against Ulster on 30 October, when McRae injured his neck, his season has been hit by disruption.
He missed six weeks with the neck injury, followed by an operation on cartilage in his knee. McRae has started the last four Gloucester games, before being replaced against Quins.
Gloucester's next fixtures are against Bath (Sat 19 Feb), Newcastle (Sun 27 Feb), Bath (Cup Sun 6 Mar) and Northampton (Sat 12 Mar).
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