Celtic reportedly have concerns over Cameron Carter-Vickers' fitness.
The USA international has reported back at Lennoxtown for pre-season.
However, it's claimed that he has yet to train with Brendan Rodgers' first team.
The Daily Record reports that there are suggestions that the centre-back has undergone surgery during the summer, albeit they play down the validity of those claims.
The former Tottenham defender has been a key player for the Scottish Premiership champions in recent years.
However, the club has had a job on its hands to try and manage his unfortunate injury record to date.
Celtic play a pre-season friendly against Queen's Park on Friday night, before trips to Cork and the Algarve in the coming weeks.
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They will then finish their pre-season schedule with a home match against Newcastle, before a tie against Ajax in the Como Cup in Italy.
Their first game of the season will take place on Sunday, August 4 at home to St Mirren.
Their crucial Champions League qualifier will take place in the middle of August.
