Manchester City manager Pep Guardiola has confirmed that Jack Grealish and Phil Foden were both left out of the matchday squad against Newcastle United on December 19 for disciplinary reasons.Grealish and Foden were outstanding in Manchester City's drubbing of Leeds United on December 15. After the match, they visited a bar and Guardiola's staff were unhappy with their condition the following day.BBC reported this with Guardiola quoted in the article as saying he keeps a close eye on his players' behaviour off the pitch.After their pictures appeared in the media, Guardiola dropped Jack Grealish and Phil Foden. The two England internationals have been key players for Guardiola's side, though it's clear the Spanish tactician expects very high standards from everyone in his squad.Speaking about their absence from the Newcastle game, Guardiola said, "I pay a lot of attention to behaviour on and off the pitch. And when off the pitch is not proper they are not going to play. So they have to be focused all the time because of the distractions in the Christmas time and everything that happens you have to still be focused."City fans may well be concerned by this development, as it continues a difficult start to life at the Etihad Stadium for summer signing Grealish.The 26-year-old looked like a world-class performer during his time at Aston V'lla, but he's not shown his best form in a City shirt so far.Despite a goal against Norwich on his home debut, Grealish has found it hard to replicate the same form that earned him the move to the Premier League champions. Guardiola recently backed the "excellent" Grealish to come good and start delivering more goals.