Coates admits the presence of manager Gus Poyet at Sunderland was a big factor.
He told safc.com: "It is a pleasure for me to be here and I am really excited for the opportunity to be part of Sunderland's team.
"It has been great since I arrived here; I know the team and the manager now so it is really good."
The Uruguay international also admitted that he cannot wait to play under Poyet, adding that the club's style of play will also ensure he hits the ground running.
"He was really good for Uruguay and he was a great player, as a manager he keeps improving after starting with Brighton where he did really well.
"For me it is really important to be here, he spoke with me and I know what he wants from me which is also important."
He added: "He tries to play good football and that is really important for me because at Liverpool it was the same style."
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