MK Dons can confirm that George Williams has suffered a rupture to his anterior cruciate ligament.
The Wales international will now begin a four to six-month rehabilitation programme.
Karl Robinson said: "It was one of the most difficult hours I've had as a manager. The kid means so much to me as a person and I rate him so highly as a footballer.
"I wish him all the luck in the world. I promised him that I will be at his first game back - I'm counting down the days."
The Club would like to wish George a full and speedy recovery.