"It is a little bit better than we thought at first but it is obviously ligament damage in the right ankle," added Martinez.
"When it is swollen it is sometimes difficult to assess but there is no fracture, we hope."
The Scot joined the club from Hamilton in July 2009, with the Latics seeing off competition from several English clubs, including Wolves, to sign the player.
"He has been sensational," stated Martinez. "Sometimes it is hard to understand the age of James.
"He plays with a great maturity and understanding and the level of performance has been so consistent for a young man it shows you the quality he has got.
"When he comes back he will be stronger and ready to make a real impact in the rest of the season."
Wigan are also set to be without defender Emmerson Boyce for up to three weeks as the 31-year-old recovers from a pulled hamstring.
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