The left back has been injured since November after having surgery on his knee.
And dressed in a bright orange bomber jacket and torn white jeans, the 24-year-old joined his pals on the pitch but was tracked by a steward.
After the official realised who he was, Mendy was left to show his affection to goalscorer Sergio Aguero.
The Frenchman gave his Argentinian pal a big sloppy smack on the cheek, before embracing his other City team-mates.
He later posted on Instagram: "Love you more than your wife bro @10sergioaguerokun."
The defending champions beat Liverpool 2-1 to close the gap on the Reds to four points in the Premier League title race.
It was not a game without controversy though, as Vincent Kompany risked being sent off with a flying, studs-up challenge on Mo Salah — but was only booked.
Pep Guardiola could have received a stage one warning as the City boss furiously hurled his scarf at the ground in protest at a foul not given on Fernandinho.
And John Stones pulled off the clearance of the season in the first half, with the scores at 0-0 — and replays showed the ball was still in play by 11.2mm.
It is not the first time Mendy has been pulled up for his celebrations — after he hobbled down the touchline to celebrate a City winner last season while recovering from a cruciate ligament injury.
It led to Guardiola asking him to curb his enthusiastic social media activity, pretty much like the activity following last night's blockbuster title clash.
Source: Read Full Article