Jack Grealish was made unavailable for Everton's Carabao Cup round of 16 match against Newcastle on Thursday due to what the club called a "breach of club discipline."
Everton did not confirm what the breach relates to, but according to multiple reports it is due to an unauthorised absence from training.
The on-loan Manchester City winger was also missed Saturday's 2-0 defeat to Arsenal, which Everton said was due to illness.
Everton beat Newcastle 3-1 to set up a Carabao Cup quarterfinal clash with Fleetwood Town. Fourth-tier Fleetwood stunned Premier League champions Arsenal in the fourth round.
Speaking after the game, Everton manager David Moyes declined to give further details on Grealish's absence.
"It was a club decision," he said. "I support the decision. It's something I don't really want to speak too much about."
Grealish, 31, is on loan at the Hill Dickinson Stadium for the second season running, but has not yet registered a goal or an assist in his seven games so far this season.
Before the Arsenal game, Grealish had started three Premier League matches in a row.
Tyrique George and Brennan Johnson featured on the wings at the Emirates, with Tyler Dibling replacing Johnson in the starting lineup for Thursday's game against Newcastle.
Last season, Grealish provided six assists and added two goals in 20 appearances before suffering a season-ending foot injury in January.
Having enjoyed his time on Merseyside, Grealish remained with Everton during his rehabilitation.
A player who cost City £100 million in 2021 began this season as part of Enzo Maresca's squad at the Etihad Stadium, featuring as a substitute in both their 3-0 Community Shield loss to Arsenal and their Premier League opener against Bournemouth, a 2-1 win.
He returned to Everton on loan on Sept. 1.
PA contributed to this report.