Translation:This guess is said by that professor.
Indeed, 'to make an assumption' sounds more natural to my ears than 'to make a guess' - I'm not a native and I'm not sure if natives agree with me on this. So my question to a native: which is the most logical infinitive missing in 'to ... a guess'. I'd say to state. Are 'to do' and 'to make' passable here?
Although there is some nuance between assumption and guess.
You'd be fairly likely to "have" a guess. You could get away with "make" a guess, but it can feel a bit off. You would never, "do" a guess and most commonly you wouldn't use any of those, instead you'd just use guess as a verb.
A: "What did you do this week?" B: "Have a guess!" A: "Well, if I had to make a guess, I'd guess you probably sat on your arse all week and watched TV.