The Denmark international had a debut to forget when after just 10 minutes he was on the receiving end of a crunching tackle which has left him sidelined for the start of the season.
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O’Riley completed a £25m move to Brighton from Celtic on Monday which makes the injury even more frustrating for manager Fabian Huerzler who had the midfielder in his plans for the season.
"It is like we expected, it is a bad injury and he needs surgery on his ankle," said Hurzeler.
"I can’t say how long or when we expect him to be back because of rehabilitation. It is a tough one for us but we will try to support him the best we can."
The Brighton manager said the tackle was “ridiculous” and that O’Riley was "very disappointed" about his setback.