The 25-year-old has joined Sporting Lisbon on loan, prompting plenty of reaction from his parent and former clubs.
Indeed, after a strong end to last season, when Dick Advocaat was able to make the Uruguayan a rock at the heart of his defence, Coates has been very poor indeed this time around.
And that's led to Sam Allardyce moving him on, on a temporary basis for now, to Sporting Lisbon.
Sunderland fans aren't surprised. They are happy, though, and have praised Big Sam for pushing out more 'dead wood.'
Younes Kaboul started this season badly but has upped his game. That hasn't been the case for Coates and the former Liverpool defender looks as though he may have played his last match for Sunderland.
It's a shame for the 25-year-old, who was viewed with some promise when he first came to England. Here's how Liverpool fans and those at the Stadium of Light have reacted to his loan move on Twitter:
blimey Coates didn't last long at Sunderland did he? #lfc— Paul Milligan (@mrpaulmilligan) January 28, 2016
Seb Coates goes to Portugal. He never really got a chance at #LFC, atrocious defending was preferrable apparently.— vruz (@vruz) January 28, 2016
How was Sebastian Coates ever considered the next big thing, he is woeful, #LFC— Jurgen Army (@liverpools_year) January 28, 2016
Coates slowly leaves the SOL nothing new there then #safc— denis hembroughsafc (@DenisHembrough) January 28, 2016
Disappointed that Coates hasn't worked out, really thought he might kick on after the way he ended last season but he's been very poor #safc— Dan (@dan_safc3) January 28, 2016
You've got to hand it to Big Sam firstly Graham then Coates and with a bit of luck Rodwell!!! #safc— Chris (@chrisbellFTM) January 28, 2016
Kone out Coates in. Seems Sam is shifting all the deadwood very slowly #SAFC— Jordan maloy (@MaloyJordan) January 28, 2016