Grealish gave Aston Villa the lead against Manchester United on Sunday with a stunning strike but Dean Smith’s side were trailing 2-1 just after the hour mark.
Tyrone Mings fired home a volley but celebrations were muted as it was clear the goal would be checked by VAR.
The visitors had already seen an earlier goal correctly disallowed for offside and Grealish told club media: “I could have put my house on it that he was offside.
“We were waiting, me, the referee and Harry Maguire were standing there and I was just saying: ‘He’s offside, I know he is’.”
But replays showed United defender Brandon Williams was playing Mings onside and the centre-back’s first goal of the season stood.
Grealish added: “You can hear the ref speaking and he says, ‘It’s a goal’, and I was like, ‘See you later, I’m off to celebrate’.
“That’s the tough thing with VAR, it’s good sometimes because it knocks out the mistakes but for me personally, I’m not too keen on it.
“It takes away that edge a bit when you score, I’m sure Ty would have loved to go off and celebrate with the fans at Old Trafford. But with VAR you never know when it’s a goal or not.”