I think no matter what the outcome was, he would be competitive against both. Weather he wins or not is another story
He loses to Frazier an ATG swarmer in Frazier is a bad style match up for a light hitting boxer like Usyk. Foreman would have difficulty catching Usyk and boxers/movers could give Foreman trouble as we see with Young, Peralta, and Morrison late in his career. But Foreman could really dig to the body and Usyk is quite vulnerable in that area. So I could see Usyk outboxing Foreman to win a decision, but I can also see Foreman stopping Usyk after hurting him to the body.
He keeps Frazier off him for 6 rounds and is well on his way to winning a boring fight. Then Frazier wakes up.... Foreman walks all over him in 3, if that. Same with Tyson.
He loses to Frazier, especially if the fight is 15 rounds. Remember what happened in the 15th round of FOTC. For Foreman I have nothing to add to this: