He, however, continued to play till the 88th minute when the former Manchester United player suffered another blow and had to be replaced.
"Paddy was starting to understand more about the club, the supporters and the general game, so it's a real shame for the boy," Moyes told the club website (www.safc.com).
"He'd just started to find his way in the Premier League... the games he was having were bringing him on and giving him confidence... we can hope getting him right for the start of next season.
"We're incredibly disappointed because we want to build a young team going forward and Paddy was part of that."
The 21-year-old, who featured in nine league games and scored a brace in the Black Cats' 2-1 League Cup win over Queens Park Rangers in September, also suffered a cut on his knee, which will delay treatment.
"They can't operate while there is an open wound, so we have to wait until that heals. It might be a week or two," Moyes added.
Sunderland are on eight points after 12 league games and in the relegation zone, sandwiched between Hull City, who are on 10 points and bottom-placed Swansea on six.
They travel to Liverpool on Saturday for their next league game.
(Reporting by Debanjan Bose in Bengaluru)
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