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And the Everton boss refused to confirm whether Grealish would be available for the Toffees' Premier League clash against the Magpies on Monday.
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Grealish was not included in Everton's squad for their 3-1 win over Newcastle due to a 'breach of club discipline'.
The forward has played five times for David Moyes's side so far this season after re-joining on loan from Manchester City.
He has started three of their Premier League matches this campaign, coming off the bench on another two occasions.
Grealish played 77 minutes in their Carabao Cup third round clash against Wolves last month and started the following three league games.
But he missed their 2-0 loss to Arsenal at the weekend and went on to be left out against Newcastle on Thursday night.
Everton revealed the news in a post on X, also confirming that Jarrad Branthwaite was sidelined due to concussion.
The club wrote: "Jarrad Branthwaite remains sidelined for a second successive game due to concussion protocols, while Jack Grealish is unavailable for selection due to a breach of Club discipline."
No official explanation of the breach has been revealed.
However, talkSPORT understands that Grealish has had an unauthorised absence from training for the Toffees.
Moyes gave nothing away when quizzed by talkSPORT for further details about Grealish's absence, declaring: "That’s for the club to announce."
And when asked if Grealish is in contention to play in the league match between Everton and Newcastle at St James' Park, live on talkSPORT, Moyes said: “I’d never give anything away regarding team talks or tactics.”
The 31-year-old has been part of their squad in all but one game this season, with that being the Arsenal clash last time out.
The club stated that Grealish missed that game due to illness.
Reacting to the new, talkSPORT's Adrian Durham said: “The fact Everton are making it clear… they want everyone to know.”
Meanwhile, ex-Premier League goalkeeper Brad Friedel added: “If you leave a player like Grealish out in a team where you struggle to score goals, you’re going to have to explain it.
“I don’t think it’s a difficult one to explain, if you miss training it’s a pretty easy decision.
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"David Moyes had to act. I think it was probably one of the easiest decisions he's had to make [if he missed training]."
Grealish didn't play, but Everton didn't need him, as they thrashed Newcastle 3-1 to reach the Carabao Cup quarter-finals.
The Toffees will now face League Two Fleetwood Town in the next round after they incredibly beat Arsenal on penalties.
James Garner, Hayden Hackney and Thierno Barry scored the goals for the hosts at the Hill Dickinson Stadium on Thursday.
Dan Burn got a goal back for Newcastle, but it was too late to spark a last-gasp comeback.