"I think we've got him at the right time and, in a few years time, he'll be worth a lot of money," City boss Gary Johnson told the club's official website.
"I think the things we do here will help him improve. He'll certainly improve in the Championship, where it's competitive week in, week out.
"Having said all that, he's not here to be a player in a year or two's time, he's here for now. He needs a good pre-season to get into our mentality and fitness. I'm sure he'll do that."