The Bulgarian striker fell awkwardly following an aerial challenge with Patrice Evra after just six minutes of his first start for the Blues.
The club confirmed they expect the 21-year-old, signed from Fiorentina for an undisclosed fee in early August, to be out for "at least five months".
Copyright (c) PA Sport 2007, All Rights Reserved.
Do YOU want to write for GiveMeSport? Get started today by signing-up and submitting an article HERE: https://www.givemesport.com/writeforgms