Sanchez was surprisingly dropped for Sundays derby at the Etihad Stadium after he impressed as Uniteds central striker against Bournemouth and Juventus.
United got off to a bad start as David Silva put Pep Guardiolas side ahead in the 12th minute and struggled to create chances against the Premier League champions in the opening 45 minutes.
As the first half was coming to a close, the camera turned to Uniteds bench and Sanchez was seen sharing a joke with Mata.
But United fans failed to see the funny side with their team 1-0 down and looking unlikely to find a way past City.
1-0 down against one of our main rivals and Mata + Sánchez are laughing on the bench. Ah youve gotta love it.
Ryan. (@Vintage_Utd) November 11, 2018
Tяυsτ №¹-Yahya (@RealistYahz) November 11, 2018
Juan Mata & Alexis Sanchez in the crowd laughing at 1-0 down says plenty about #ManUtd
Michael Sullivan (@Sullivan87M) November 11, 2018
That @Mata and Sanchez smile just shows how those players dont give a fuck
GGMU (@RooneyTheLegend) November 11, 2018
Sanchez and Mata laughing on the bench… oh how the Manchester derby has changed..
Andrew Bustamante (@AndrewBusta11) November 11, 2018
Lmao Sanchez and Mata having a laugh
Master Chief (@luke_shawarma) November 11, 2018
@ManUtd Why are Mata and Sanchez laughing on the bench? We are losing 1-0!! Wtf!!!
bob (@bobfortheworld) November 11, 2018
I love jokes so I would love to know whats so funny being 1-0 down to your rivals, looks like Mata and Sanchez enj… twitter.com/i/web/status/1…
FergieTime (@SSIIIIUUUU) November 11, 2018
Also fuck off Mata and Sanchez laughing away on the bench.
ShaneM08 (@shanemufc20) November 11, 2018
WHY THE FUCK ARE MATA AND SANCHEZ LAUGHING?
Fosu-Mensah_Fan (@MasterB12702792) November 11, 2018
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