The Uruguayan moved to the Stadium of Light from Liverpool on a season-long loan deal on transfer deadline day.
Poyet rates Coates highly, having recommended his compatriot to Liverpool in the summer of 2011 when he was then in charge at Brighton & Hove Albion.
"I know Sebber, I recommended him to England in the beginning when he came to Liverpool. He was a 6'3" centre-half, calm on the ball and with aggression," Poyet told the Sunderland Echo.
"He's a good player. Slowly, he's going to have to start competing with Wes and John and when he has the chance to play in any game - even in the cup - he needs to perform.
"I think it's a great chance for him as well and a great challenge. We needed a centre-half too, so it works both ways in helping each other."
Coates has played just six Premier League games for Liverpool after suffering a cruciate ligament injury.