Mundle took to social media on Saturday night to break the news of his setback, which is a major blow for the Black Cats just a month out from the start of the Premier League season.
The former Tottenham youngster twice injured his hamstring last season and suffered a recurrence of the problem in Portugal week. It’s not clear at this stage how long he’s expected to be sidelined, but he won’t be ready for the visit of West Ham.
“He had surgery today, so we will have to wait,” said head coach Regis Le Bris.
“It’s a shame because he worked well during the summer and was ready, but we will support him.”
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Earlier on Saturday, Mundle posted on Instagram: “Devastated to be here writing this message and words can’t fully describe the emotions attached to this setback.
“Sometimes it’s hard to find the positives in situations like these but without a doubt Ill be back better physically.
“Surgery was a success and the road to recovery begins now. Although it has been a difficult period for me over the last few days, this is only a minor setback and I will be back better than before.”
In Mundle’s absence, Sunderland drew 1-1 with Sevilla on Saturday night.
Enzo Le Fee gave the Black Cats a first half lead, but the Spanish side levelled deep in stoppage time.
Simon Adingra, Noah Sadiki, and Chemsdine Talbi all made their debuts for Sunderland. The trio started the game alongside fellow summer recruit Habib Diarra.
Sunderland are next in action when they take on Sporting on Monday night.
The Black Cats are on the verge of making their seventh signing of the summer, with Armand Lauriente having landed in Portugal on Saturday to undergo his medical and finalise his move from Sassuolo.