With second-choice goalkeeper Keiren Westwood currently recovering from shoulder surgery, 27-year-old Ustari will go straight into the squad for Wednesday's League Cup semi-final second leg at Manchester United.
In Westwood's absence, the only other goalkeeper at Sunderland's disposal apart from first-choice Vito Mannone was untested youngster Jordan Pickford.
Speaking shortly before the transfer was announced, Sunderland manager Gus Poyet said: "I hope he (Ustari) gets over so we can go tomorrow (Wednesday) 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."
Sunderland lead United 2-1 ahead of Wednesday's game at Old Trafford.