Foster would KTFO Beterbiev. If you can't stand in front of Callum Johnson without eating a wicked left hook, you're not gonna do it to Foster, and unlike against Callum, if Beterbiev moved off and regrouped, he'd simply be moving to Foster's preffered range. Beterbiev does have a good chance though, as he's stronger, is a huge puncher and has amazing fundamentals. Foster's chin was far from legendary, and so Beterbiev can definitely win by KO, but I struggle to see him actually getting the KO without eating enough to either put him off trying, or put him to sleep.
The only thing I would add to this is that I have serious doubts that Beterbiev is a 'huge' one punch hitter. Both Yarde and Smith took plenty of shots before they eventually succumbed. Between the two, Foster hits significantly harder with single shots and I would favour him to get the job done.
Beterbiev is a 1 punch hitter quitter. When he loads up and lands the fight is basically over. He prefers to touch, touch, touch and keep you from establishing any rythm. Beterbiev would stop Foster, not from loading up. But the short shots on the inside would wear him down. Beterbiev KO14.
I think Foster would destroy him. His jab was hard, he was adept at landing hard rights when fighters try to get on the inside. He landed uppercuts with ease and worked the body fromthe outside. He tied up opponents easily when they got inside. Beterbiev is a good fighter, strong and durable but he has to get close to do damage would be his undoing. Would be a great fight with Bob breaking him up and stopping him in about 8 rounds.
It's interesting to me how underrated Foster has become at 175 .... the man had the size, skills, jab, speed and exceptional power to compete w anyone at that weight .. he didn't carry the advantages up to heavyweight but his four big losses , Ali/Frazier/Terrell/Folley were to serious heavyweight of the time and the Jones loss was a case of a last minute substitute going up against a damn good fighter with far more experience ... I don't really know how good Beterbiev is although he seems very tough and cagey with serious power but there are also a lot of rumors about him and drug enhancement as well ... I guess time will tell but few 39 year old fighters keep improving in reflexes and speed ...
Saw Bob Foster defend his title in person in 1969...champ from 1968 to 1974....Foster was a beast at Lt Heavy....he wins by murder...