Jonathan Spector faces up to six weeks on the sidelines after he has surgery on his broken cheekbone.
The American international midfielder picked up the injury last week in a behind-closed-doors training match against MK Dons during the club's pre-season training camp in Ireland.
The 27-year-old will go under the surgeon's knife tomorrow (Wednesday).
"Jonathan Spector goes in for surgery tomorrow and will be out for six weeks," confirmed boss Lee Clark.
"But there is some positive news with Will Packwood (broken leg) and David Murphy (knee). They are both doing terrifically well with their rehab and they will probably be back in with the group round about the Watford game in terms of training."