The source Rob Holding's blow-up with Crystal Palace boss Oliver Glasner has been identified as a row earlier this season.
Holding has been frozen out of senior training and also missed the club's photoshoot last week.
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BBC Sport says the former Arsenal defender's fallout with Glasner came after the defeat against West Ham on August 24.
Holding was in the squad, but didn't get on the pitch.
And it is said that he challenged a coaching demand for a running session after the final whistle on the Selhurst Park pitch.
The defender hasn't been involved in the first team ever since.