Roberts signed for Celtic on an 18-month loan deal last January and came back to haunt his parent club with a wonderful finish.
The young forward picked up the ball on the right wing, cut inside, danced past the City defence and smashed past Willy Caballero to give the Scottish side the lead.
The 19-year-old signed for Man City in July 2015, playing just three times for the club before re-joining Fulham on loan and then current side Celtic.
The quality of Roberts' goal is likely to have caught the attention of Guardiola and the Englishman may become a regular under the City boss next season, given his tendency to give youth a chance.