The Blues originally decided the left back would undergo a course of physiotherapy instead of surgery, which can be used to deal with less serious ACL tears.
However it is thought a scan earlier this week showed an issue, and led Chelsea doctors to change their minds and opt for surgery.
Chilwell had the operation yesterday, and posted a message to fans after surgery.
He uploaded a photo onto Instagram, giving a thumbs up while holding crutches.
Chilwell said: “Just wanted to let everyone know i am on the way home from a successful op.
I feel very positive and motivated to work like a beast to get back on the pitch with my boys soon and help this great club win more trophies.
“Thank you all for the support and nice messages.”
Plenty of football stars left messages of support.
Liverpool captain Jordan Henderson said: “Recover well mate.”
While the Three Lions wrote: “Wishing you a speedy recovery, Chilly!”
And City star Jack Grealish posted two hearts.
Chilwell was having a fine season, scoring three goals and contributing one assist.