Ustari joined the Black Cats after cutting short his contract with Spanish side Almeria.
Poyet said: "You need to get used to the contact in English football and we have been talking to Oscar a lot about that.
"If you touch the goalkeeper in Spain it is a foul.
"There are plenty of fouls on a goalkeeper in England which are not given.
"It is unlucky for the goalkeepers, but you need to be used to that."
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