Sunderland completed the signing of Mika after the goalkeeper's move from Boavista was delayed by a "technical issue" at the Portuguese club's end.

The 25 year-old has signed a two-year contract at the Stadium of Light and is David Moyes's 10th signing since succeeding Sam Allardyce as manager.

In a statement on its official website, Sunderland explained FIFA had ratified the move for Mika, a former Portugal Under-20 and Under-21 international, because "both teams fulfilled their commitments" prior to the 31 August deadline eight days ago.

"The deal has been ratified by FIFA, after Sunderland completed all of the necessary protocols in advance of the transfer window closing," the statement read. "A technical issue meant that Boavista were unable to complete their requirements by the 11pm deadline on August 31, however FIFA are satisfied that both teams fulfilled their commitments and the transfer has now been approved."

Sunderland returns to Premier League action against Moyes's former club Everton on Tuesday (AEST), while Mika will contest goalkeeping duties with Vito Mannone ad Jordan Pickford.