Phillips loan spell ends at the end of the month and his Bristol Rovers contract expires in June, Phillips has expressed his desire to sign for the Railwaymen permanently when his Rovers deal expires.
Gradi told the Chronicle "Steve has had a problem with his shoulders for a while. We have listened to the medical advice and he will undergo an operation on one of his shoulders. He will receive rehab on the other one.
"He wouldn't have had two at once anyway and we are trying to avoid him having surgery on the right one if we can.
"Steve has played through the pain barrier for a while and I've asked him to carry on if he could. He has done that to help us with the minimum of fuss.
"That is him finished with us now but hopefully, if things can be sorted out with Bristol Rovers, we could welcome him back next season as our player. We'll have to see what happens because nothing is ever definite until it is done."