I think we'll see Greenwood rotate between striker, right forward and bench as we have a lot of (quality) players to give minutes to instead of running the same ones to the ground. I still think Greenwoods future is at #9 and some of Ole's comments suggest the same.
However, I also think Tony offers something different to both Greenwood and Cavani. The latter has superlative movement and eye for goal, while the former is clinical shooter with both feet. But neither of them has the hold-up play with his back to the goal, or the close ball control that Martial has. If he finds his 19/20 shooting boots he'll be difficult to dislodge from the top, because he can bring other players into the game more than our other #9s. That's a big if though.
In any case it's nice to have 3 entirely different options to choose from and try out in games when one isn't working.