Sunderland fan here. Trying to explain what to expect.
I know fans will think he played for England, big signing for Man City and maybe a fresh start etc. I understand it but we will be very very surprised if he turns out well for you. He has a bit of natural talent (a bit, he was hyped up quite a bit at Everton but was never really that good - more promise than anything). He is not the worst technical footballer to play for our club. He is the worst signing in our history (even worse than mistakingly signing Milton Nuñez, a Honduran dwarf who fell over the ball on his debut and only appearance). He is also one of the worst professionals in history.
Why? Complete lack of application. When he was on the pitch he had an ability to hide from the ball, to make himself unavailable for a pass, would rarely make a tackle, wouldn´t track runners, would not compete for a ball, no fight or will. He was a passenger. It may be that he has mental problems, which we could understand, except he would blather on about how he was fit and not given a chance. Then he would be given a chance, not be fit and not even make a professional effort.
He was on 70k a week and couldn´t be bothered to apply himself. Made no effort to be a professional footballer. I believe that any physically able football fan, given 70k a week, would try. Jack didnt try, and then he blamed everyone but himself.
Expect a litany of excuses. Don´t expect to win a game with him on the field. We did once - 4-0 and all after 4 years of him not winning a game, but we only won that game because our team basically bypassed him the whole match. Welcome to the worst excuse for a professional footballer we have ever seen.