Quizzed afterwards about his selection, Guardiola said the players he picked “deserved” to play.
“It was not rotation, no,” the City manager told the BBC. “I decided on this team because they deserved to play today, these guys and not the other ones.
“At Christmas time 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 we have to be focussed all the time because of distractions at Christmas time and everything that happens. You have to still be focussed.”
Grealish and Foden were both unused substitutes as City made it eight wins in a row to move three points clear at the top of the Premier League table.
City are one of the few clubs who are yet to have a game postponed this season because of COVID-91 problems.
Players have been reminded to keep socialising in groups to a minimum over Christmas in an effort to deal with the coronavirus spike in the UK being fuelled by the highly contagious omicron variant.
Guardiola was forced to cancel his usual news conference on Friday after returning an “inconclusive” lateral flow test.
He was cleared later in the day after taking a PCR test and was able to take his place on the touchline against Newcastle.