Mignolet's move to Liverpool was confirmed this evening and Di Canio also said that the deal would allow the Black Cats to strengthen their squad ahead of next season.
The Italian told the Sunderland website: "Simon is a fantastic footballer, a fantastic professional and a fantastic lad and I would like to personally thank him for his contribution to the team last season.
"He was crucial to what we achieved and he goes with the very best wishes of everyone at the club.
"I would like to assure our fans that there is a huge amount of work going on behind the scenes."
Mignolet made 90 appearances for the Wearside club since joining from hometown club Sint-Truiden in 2010.