He also previously spent time training with Villa’s youth team as a punishment when Remi Garde was in charge at Villa Park, after reports he visited a nightclub following a 4-0 loss at Everton.
“Unfortunately it has happened again. We expect our players to behave in a professional manner at all times, so it is something that we don’t tolerate,” added Di Matteo.
“It’s important that he understands what he has, and he needs to grow up very quickly.”
The Solihull-born former England Under-21 player signed a new four-year deal with the Championship club on 7 September.
Villa owner Tony Xia contacted Grealish directly via the social media platform Twitter on Wednesday, saying: “As I said before, focus every minute, on and off the pitch, choose the right people to be with.”
Xia added: “It’s not a warning but just elder brother’s talk. I trust Jack can really learn and be a man.”