Southampton are expected to confirm the signing of 18-year-old Romeo Lavia from Manchester City on Wednesday.
The deal is believed to be worth an initial £10.5m with a further £3.5m in add-ons and a 20% sell-on clause.
City also have a buy-back option for the defensive midfielder, who signed from Anderlecht in 2020 and made his senior debut for the Blues in the League Cup last season.
"My Manchester City journey has come to an end," Lavia posted on social media.
"I've developed into an even better player, person and it was also an honour to make my debut for this massive club.