The 19-year-old enjoyed a breakthrough season at Stamford Bridge, making 11 appearances in all competitions and impressing in the centre of midfield.
Gilmour’s progress was halted by a serious knee injury in July, which ruled him out of the rest of the campaign.
When he does return to action, he will be wearing the No.23 shirt, instead of his old No.47.
Michy Batshuayi’s exit on a season-long loan move to Crystal Palace made the number available.
The Belgian striker couldn’t resist poking fun at Gilmour for taking his shirt, writing on Twitter: ‘Oh… I thought you were my guy @billygilmourrr 😂….. Love bro make me proud 💙.’
To which the teenager replied: ‘You’re my bro always Michy 🙌 🤣.’
The English boss will be looking to better his side’s fourth-place finish last year and get closer to the likes of Liverpool and Manchester City.