MILLWALL midfielder Nicky Bailey was substituted at half time against Norwich on Saturday after suffering a back spasm.
Bailey had been sidelined since September with a calf problem but the club have confirmed he did not suffer a recurrence of the same injury.
The 31-year-old experienced pain in his back and will now be monitored by the Lions' medical department.
"The reason I put Bailey in was because he helped us so brilliantly last year to fill gaps in our defence," said boss Ian Holloway.
"That's what I was hoping would happen on Saturday.
"Unfortunately I had to bring him off at half time because he's had a reaction to all the work he's done and the twisting and the turning.
"It's nothing to do with my physios – they're brilliant – and this club has a fantastic record of keeping players fit and getting them fit.
"The stats show that."
Shaun Cummings and Stefan Maierhofer resumed full training this morning, while Paris Cowan-Hall is also being eased back into the group.
"Maierhofer gives me the opportunity to play the ball from back to front," added Holloway.
"He really does cause all sorts of issues and that brings Diego Fabbrini into things when he plays in the pocket.
"We've also missed the legs of Lee Martin and Paris, who the fans haven't even seen yet.
"Cummings had been excellent and we were starting to look solid at the back with him at right back.
"I've had to play Jimmy Abdou there and then move Alan Dunne when we needed a settled back four, which is vital."