McCarthy, who turned 23 on Monday, went under the knife at a London hospital earlier this week.
The former England Under 21 stopper could miss the rest of the Premier League season, though it is believed Royals are optimistic he could be back sooner.
Reading Head of Sports Medicine Luke Anthony explained: "Understandably supporters will ask how long Alex will be out, and it's difficult to put a time frame on his return.
"The most important thing is for Alex to make a full recovery and return in optimal condition. If that allows him to return before the end of the season then that is something we will try for."
McCarthy damaged his shoulder when he crashed into a goalpost while saving a Djibril Cisse shot in the dying seconds of Reading's 1-1 draw at QPR in early November.
Adam Federici, 27, has been playing between the sticks ever since and the Australian international is poised to start this Saturday when Reading travel to Southampton for a Premier League basement battle.
Jimmy Kebe could also be involved, with the winger facing a late fitness test as he recovers from a groin problem.
Central midfielder Danny Guthrie is likely to be in the squad despite picking up a knock playing for the reserves in a 2-0 defeat by Stevenage earlier this week.
But Russian striker Pavel Pogrebnyak looks set to miss the trip after sustaining an injury in the same reserves game.
If Pogrebnyak is sidelined, highly-rated Academy teenager Dominic Samuel could force his way on to the bench for the clash at St Mary's (3pm ko).
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