The England international got the nod over Jeremy Doku on the left wing and was given the chance to impress Pep Guardiola.
However, while running, he was clipped by Diogo Goncalves and collapsed to the floor in agony.
He immediately pointed to his groin and signalled to the bench that he needed to come off.
Doku came on for the former Aston Villa man, who couldn't hide his devastation as he was removed at the Parken Stadium.
It's not clear what the extent of the groin injury is, but it'll be a huge blow for Grealish with Euro 2024 on the horizon.
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The 28-year-old hasn't had the best season for the Citizens this year, scoring just three goals in 25 appearances, so this was a chance to stamp his authority on a game and force his way back into the regular starting XI.
Man City won 3-1 on the night, with Kevin De Bruyne opening the scoring before Magnus Mattsson equalised with a lovely curling shot, but Bernardo Silva was on hand ensure City had a lead to take back to the Etihad, and Phil Foden then made things comfortable.
Meanwhile, talkSPORT's Stuart Pearce was on co-commentary, and he watched Grealish go down and described the events live on-air.
He said: "I think his groin's gone.
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"He's holding his groin. He was clipped on his foot, but he pulled up really quickly, looked up to the bench and the look on his face said this is something a bit more serious than a clip.
"Somehow, in that contact, his groin has gone."
The one positive was that Grealish managed to walk off the pitch unaided.
But the way he pulled up suggests this could be quite serious, which would be a huge blow for the winger.