"Its not good news, he's torn ligaments in his elbow. We're just finding out if it'll require surgery or whether it can be put in a brace for the time being but its a significant injury," Moyes told reporters on Friday.
"The doctors' first feeling is that it would be a minimum of three months (out). We definitely need to look for another goalkeeper because we only have a young 'keeper after Jordan Pickford. He now has his chance to show what he can do."
Sunderland have added Papy Djilobodji, Paddy McNair, Donald Love, Adnan Januzaj and Steven Pienaar to the squad in the transfer window. Moyes confirmed the club's interest in Seville midfielder Vicente Iborra.
Sunderland lost their opening two league matches of the season to Manchester City and north-east rivals Middlesbrough.
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"If we can go to Southampton and get a point it would be a good result, but we want to go there and win," Moyes said.
"I thought Southampton played really well at (Manchester) United and thought they were unlucky. The club have a real solidity about them."
(Reporting by Ian Rodricks in Bengaluru, editing by Ed Osmond)
