The midfielder was dropped for Saturday's 3-2 defeat to Watford after being pictured in a nightclub following the previous 4-0 loss at Everton
Press Association Sport understands Grealish was expected to link up with the under-21s again after manager Garde last week insisted he needed to act more professionally and issued the punishment as Villa battle relegation from the Barclays Premier League
Grealish, 20, was warned about his off-field behaviour in April by then Villa manager Tim Sherwood after he was photographed apparently taking nitrous oxide.
In June, Villa said they would speak to him after photographs were published appearing to show him lying in the street while on holiday in Tenerife.
Grealish made his Villa debut in 2014 and has made 12 appearances, scoring once, this season.
Defeat to Watford left Villa seven points from the safety line but Garde could ring the changes, specifically in defence, for Saturday's trip to Southampton
He said afterwards: "It is not always individuals, it is sometimes collective
But also as individuals, for sure, they have to improve their level."
Meanwhile, former boss Sherwood has backed Villa to survive despite their desperate situation
Sherwood was sacked in October after six straight defeats and replaced by Garde earlier this month but he believes they can stay up.
"I wish them well," Sherwood told the Daily Telegraph.
"It's difficult when you lose your three best players and you finished 17th in the Premier League the previous season
It was always going to be a tough task.
"The club is always going to be bigger than any individual
I respect their decision
I believe they will stay up this season
There were some good times in a very short period."
Source : PA