I'd like to share a photo shot by a pal on another forum:
He used 200mm f2 to do this shot: tell me if any shorter lens can ever achieve this effect:
hint: subject distance: 17.6m
35 1.4? pfff, not even close
Contrary to popular belief, in order to get most background blur (isolation), you shoot head/shoulder with wide angle (35), half body with medium (50/85) and full body with telephoto (>200), not the other way around.
Wide for full body, tele for close up just gives you flat and boring snap shots.
The OP asked a very general question. With your very nice posted image, taken with a 200mm lens, another focal length prime will not create an identical image. The background blur can be less or more depending on the working distance, focal length, aperture and subject distance from the background. If you hinge on a 200mm then the only way to create a simular isolation is to go closer with a wider lens or further back with a longer lens. If one goes wider, then a larger aperture will be needed to create a clean background.
A 35mm f1.4 and 85mm f1.2 are both perfectly capable of doing this, although the 35mm is better suited to "head and shoulders" portraits and not "head and toes" portraits. The wider lens will be a lot closer and cause a different look to the 200mm image. There will be a different look from the amount of compression difference between the 200mm and 35mm, but that's an entirely different subject.
I regularly use a 35mm f1.4 and 85mm f1.2 for my professional wedding work and it's very very easy to melt the background, but tricky to get a precise point of focus. That said, my 24-70L is very good at melting backgrounds too. At 70mm @ f2.8 it's min focus distance is very close and more than capable of making clean backgrounds.