The full extent of the injury is still being assessed but there are fears that the on-loan Tottenham youngster could even be set for as long as six weeks on the sidelines.
The news is a blow for manager Chris Hughton, who has seen his side score just twice in four league games so far this season.
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"He is being assessed. He will certainly be out for a period of weeks, a month-plus rather than short of that. It is disappointing for us.
"He will be a big loss because he is a young player of fabulous potential. It was a big move for Harry to come here, an exciting move. I feel for him at the moment.
"It is a real disappointment, especially when he was just up and going. He has settled in very well. We just hope it is a shorter period out rather than a longer one."
The 19-year-old made an impressive impact after coming on as a substitute in his debut against West Ham, and his goal-scoring record at youth level and in previous loan spells at Millwall and Leyton Orient seemed to bode well for his future at Carrow Road.