The Germany international was outshined by City's loanee Patrick Roberts, who opened the scoring for Celtic after four minutes.
Sane, however, could not do much right, adding nothing to the home side's attack, and the 20-year-old was a magnet for Twitter haters as a result.
Leroy Sané is one of the worst wingers I've ever seen play football— Jamie (@jcooke89) December 6, 2016
Leroy Sane is a terrible player, what a waste of money he is #UCL— Lewis Blyth (@lewisblyth93) December 6, 2016
Leroy Sané is painful to watch— Donald Love (@denholm1995) December 6, 2016
Leroy Sane being worth £40m means Zaza should be worth about £50m. Joke player.— Liam⚒Hunter (@Liam_Hunter93) December 6, 2016
oh my god leroy sane is a woeful footballer— Masai Lincoln (@MasaiLincoln) December 6, 2016
That Sane has been terrible everytime ive seen him— Leroy (@LeroyNUFC) December 6, 2016
Pep should've never bought Leroy Sané considering he will have Patrick Roberts next season, who looks better. #MCICEL— Arturo (@ArturoNotVidal) December 6, 2016
While the former Schalke star had his fair share of critics, some were optimistic about his future.
We've seen glimpses of Leroy Sane's pace and grace on the ball but lacking that final finish - you can't fault his intent #MCFC— Emilio Galantini (@SportsJEmilio) December 6, 2016
I see what some people see in Leroy Sané. Hopefully he gets some more PL game time soon.— Barry Glib (@DormanEra) December 6, 2016
Leroy Sane is lighting fast and very strong for a winger. Can't wait till he gets that final touch and finish.— Brad Twevens(12-9) (@CAMckay4) December 6, 2016
And some didn't care about Sane's performance at all...
Leroy Sane i fancy him— Charlotte (@crazycaramelxx) December 6, 2016
I just wanna play with Leroys hair— Charlotte (@crazycaramelxx) December 6, 2016
What did you make of Sane - is he destined to be a future star or a big-money flop? Let us know in the comment section below!