Carlton star Charlie Curnow has been sent for scans after leaving the field midway through an open training session.
Curnow had the area around his right knee assessed by medical staff after pulling out of match simulation at Ikon Park on Saturday.
The two-time Coleman medallist had been on a modified program over summer after undergoing knee and ankle surgeries in the off-season.
Curnow was only scheduled to take part in half the match simulation and looked good early, kicking a goal and marking well.
But the latest setback could put Curnow in doubt for pre-season hit-outs against St Kilda and GWS.
The Blues start the home-and-away season against Richmond on March 13.