"The club would like to place on record its thanks to Roberto and wishes him well for the future."
Sunderland, who face Southampton on Saturday, are second bottom of the Premier League.
Sunderland, who sacked manager Paolo Di Canio in September and replaced him with Gus Poyet, have recruited 14 players since the end of last season with many only on the fringes of the first team.
(Reporting by Toby Davis; editing by Ed Osmond)
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