Cameron Carter-Vickers played his first game since picking up an Achilles injury in October as Celtic drew 1-1 with Shelbourne in their first public pre-season friendly.
The injury picked up by the 28-year-old United States centre-half in a Europa League win over Sturm Graz meant he missed the World Cup as well as Celtic's Scottish league and cup double.
"At first, I just thought I had a bit of cramp in my calf," Carter-Vickers told Celtic TV.
"Pretty soon after that, when the physios saw it, they thought it was a rupture in the Achilles, which it turned out to be. Obviously at that point, you know it's going to be a long-term injury.
"For me, it was just about understanding that's the situation you're in and just working towards getting back in the best shape and the best way possible.