The former Charlton and Middlesbrough playmaker has not featured since the end of January due to the problem.
"Nicky is doing pool work to retain his fitness while he recovers from the back injury," Lions physio Bobby Bacic told the club website.
"The injections he had last week have helped calm the pain down and Nicky will have a review with the surgeon this week to check on his progress."
Bacic also confirmed goalkeeper David Forde should be back in training later this week despite dislocating a finger at Barnsley on Saturday.
"Luckily he managed to remain very calm and allow the finger to be relocated on the pitch, which is a very painful procedure," added Bacic.
"In a way it adds further insult to injury as it is just as painful as the original force needed to dislocate the finger in the first place."