The 27-year-old Argentina international will remain at the Stadium of Light until the end of the season after he and Spanish club Almeria agreed to rescind his contract.
Speaking minutes before the deal was confirmed, Black Cats head coach Gus Poyet said: "I hope he [Ustari] gets over so we can go tomorrow to the game relaxed and having him on the bench to cover us just in case.
"We could be in a very difficult position. The kid [Pickford] has never played and being in that situation - imagine if we got to extra-time and penalties.
"Yes, he could be the hero or it could be devastating for him as well."
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