Alistair Johnston is facing up to six weeks out at Celtic after suffering an injury against Kilmarnock.
The Canadian international was forced to leave the pitch at half-time during the 5-1 Rugby Park victory.
Assistant manager Shaun Maloney initially said it was a precautionary measure after Johnston experienced a "niggle."
He hoped the defender would not miss important upcoming matches.
However, it is now understood that Johnston's absence could potentially last until the September international break.
A six-week lay-off would mean missing nine games, including the Champions League play-off against Austrian champions LASK.
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He would also miss the opening League Phase match of European competition and the first derby of the season at home against Rangers.
Last season, Johnston missed 57 matches due to hamstring injuries in the wake of a summer of disrupted rest due to Canada's participation in the Gold Cup.
He returned for Celtic's final five league matches, making a significant contribution to the winning run that resulted in a dramatic final day Premiership title victory at the expense of Hearts.
Following a summer of limited rest after reaching the World Cup's Round of 16 with Canada, Johnston joined Celtic's pre-season training in late July.
Everton have shown transfer interest in Johnston, with Martin O'Neill confirming contact from the English Premier League side regarding a potential move.
Teenager Colby Donovan has been O'Neill's primary alternative to Johnston in early-season matches, with Anthony Ralston also available to step in.
