His only chance to KO/TKO Johnson is if he's able to land those devastating left uppercuts to the body. Just like how he knocked Andrade out. So I'd say 15-20 %.
i voted 30-40 but actually i think those are the chances for Lucian stoping Glen by KO or TKO (with at least one KD); Johnson is durable and Bute is cautious that's why his bodyshots must be very well timed so that he won't be caught by many dangerous counter rights;i see the first few round being very poor in events, middle rounds a chess match- here Bute might hurt Johnson but Glen's great experience might save him and give him another chance at a final assault in the late rounds i hope Lucian wins by KO/TKO, i believe it will happen somewhere between rds 8-10
Glen Johnson is one tough warrior, I see Bute outworking him. But 25% chance KO but I believe it will just be a Wide UD win for Bute. He will outwork Glen but not stop him, He has too good of a chin. Body Shot is best bet. Lucian Bute wins by UD ( 119-109, 118-110, 119-109 for your winner by Unanimous Decision, And still the undefeated IBF World Titleholder, Luuuuuuuuuuccccccccccciiiiiiiiiiiaaaannnnn Bbbbbbuuuuuutttttttttteeeeeeeeeee!
As mentioned, the body is the key. If I was Bute I'd be jabbing to the head and throwing hard mostly to the body all night long. The straight right to the body for 6 rounds to take the gas out of Johnson's already diminished tank. Then I'd switch to the more damaging left hook, if he does that, I think he can put Johnson away and make a huge statement.
Johnson has not been knocked out for over a decade (since Hopkins did). So, I'll give Bute 0 chance to KO Johnson.