United can confirm defender David Mirfin will miss the start of the season.
The centre-back had the cartilage in his left knee trimmed in an operation on Friday morning.The former Watford man had been struggling with a knee problem for the last few days after feeling some discomfort, which had worsened.
He will definitely miss the away League One opener at Swindon Town in three weeks' time.
Meanwhile, Terry Hawkridge will sit out the away friendly at his former club Gainsborough Trinity on Saturday.
The winger has a slight knee problem and will be assessed over the next few days, but it is hoped he will be fit again ready for next week's friendlies.
Elsewhere, there are no fresh injury worries heading into the Gainsborough clash.