Gregory managed Villa between 1998 and 2002 and was an assistant at the club between 1994 and 1996.
The 69-year-old, though, was in charge of Crawley when Grealish was playing for Notts County on loan from Villa during the 2013/14 season.
Gregory has since revealed how he managed to get the £100million Sky Blues star substituted off during a match.
"I actually got Jack Grealish taken off once," he told the Undr the Cosh podcast.
"He was on loan at Notts County from Villa so this would be about 2014. He must have been about 18/19 at the time.
read more on jack grealish
Grealish has time of his life as he stares down X-rated gestures from ultras with smile
Jeremy Doku gets last laugh on Antony with dig and agrees with Jack Grealish epic troll
"I'm in the dugout and whenever he came near me, I said', f****** Jack Grealish. Aston Villa. F****** rubbish. No wonder they've f****** sent you out to Notts County.'
"I'm right in his ear, 'no wonder they don't f****** want you up there'. Getting into him.
"They take him off and he went home and told his dad and his dad spoke to one of my friends.
"He said, 'hey, what about your mate Gregory having a pop at my Jack'. So my mate said, 'took him off though, didn't they?'"
Most read in Football
Long-standing Florentino Perez rule may be why Jude Bellingham took no.5 at Real Madrid
Ferguson: I sweated more during my driving test than any Premier League match for Brighton
New Ferguson deal, Maddison out of England squad, Ratcliffe to Man United update
Zidane only needs one word to describe Messi, who is awestruck by ex-rival's gift
Grealish would leave the pitch on 66 minutes in a 1-0 defeat to Crawley back in 2013.
The England international has since disputed Gregory's foul-mouthed rant towards him, as he responded on social media.
"John Gregory - Episode 1 of things that never happened... what a clown," Grealish wrote on Instagram.
Going head-to-head against Crawley a second time, the midfielder later got revenge over Gregory as he played the full 90 minutes in a 1-0 victory.
Grealish managed 39 appearances while on his temporary switch, netting five times in the process.
His time with the Magpies clearly stood him in good stead as he later broke into the Aston Villa first-team.
The Solihull-born ace would eventually go on to join Manchester City in a £100m deal and continues to be a shining star under the stewardship of Pep Guardiola.