Before joining Sunderland, Adam Johnson assured Martin O'Neill that his "party lifestyle" had been consigned to history. As Johnson prepared to face his former employers at Manchester City on Saturday O'Neill acknowledged that he challenged the England winger about his off-field routines before paying City £10m for him this summer.

"I heard a few things about his lifestyle when I was trying to sign Adam," said Sunderland's manager. "I did mention it to him and he assured me that those things were mainly in the past and he has settled down now but the proof will be in the pudding. I think Lee Cattermole once told me the same thing. And I believed him then, bigger fool me!"

If Johnson's reported fondness for bars and nightclubs belongs largely to a past sometimes shared with his one-time Middlesbrough team-mate Cattermole, the 25-year-old has not forgotten the arguably harsh treatment he received from Roberto Mancini, City's manager.

"Has Adam a point to prove?" O'Neill said. "Yes. He will relish going back. He will want to do well and I hope he comes up big in the game. Adam has great talent and terrific ability. He always thought he could hold down a place at City but he did not play often enough. It frustrated him and I hope we can reap the benefit."

Johnson trusts that a fresh start at Sunderland will enhance his England chances. "Before leaving, Adam will have thought that, with the big schedule City have in the Champions League this season, he may have got a run in the Premier League," O'Neill said. "That must have given him a dilemma. And leaving a big team who have won things and are capable of winning more isn't easy.

"But Adam still wanted to leave. His position with England became very important and Roy Hodgson has said it would be better to see his England players playing regular first-team football. I do think that played a part in Adam's thinking. England is very important to him. That helped sway his mind and was good news for us."

A niggling thigh injury has meant Sunderland are yet to see the best of Johnson but they will need him at his peak if last season's performances against City – when O'Neill's side won at home and drew away – are to be repeated. Especially as Cattermole, their influential captain, is suspended on Saturday.

"I hope Adam can lift us with his ability," O'Neill said. "He does voice opinions but he's pretty quiet – although that may not be the case when he's with Cattermole. With Cattermole he might turn into a bit of a chameleon."