Injured Harper sidelined for 12 weeks
Harper sustained the injury in a challenge with Everton striker Jermaine Beckford during Newcastle's 1-0 victory on Saturday.
"Following a shoulder ligament injury sustained in United's game against Everton on Saturday, Steve Harper has undergone a successful operation on Wednesday," read a statement on the club website.
"It is likely that Steve will be unavailable for selection for a period of around 12 weeks."
Harper, 35, is likely to be out until the middle of December.
His absence offers an opportunity for stand-in keeper Tim Krul who replaced Harper at Goodison Park and in Wednesday's Carling Cup victory over Chelsea.