10-15 G/A in all comps, good facilitating contribution, and giving us some tactical flexibility upfront are the realistic expectations IMO.
I would be inclined to start the season with him upfront as our starting #9. According to some people on Twitter who have watched him extensively and know a lot more about him than United fans do, he mentally isn't ready to just be a bench option at this stage of his career. Even before we were linked with him, I also thought the move doesn't make sense from the player's point of view, as with Hojlund already in the squad, we needed a striker that was willing to be 2nd choice without kicking up a fuss, like a Joselu or Choupo-Moting, for example. I feel like if we give Zirkzee a real chance up top from the get go, and then he gets benched for Hojlund, he'll take it much better than if he was just warming the bench from the beginning. Overall minutes shouldn't be a problem, though, so there will be a lot of play time available for him IMO, just not as a guaranteed striker, unless he completely hits the ground running for us and becomes undroppable.
I'm really interested to see how his first season goes at the club. I think we should be patient with him and many other current players as well.