In a vacuum I'd say he's a shade better than Martial (his record is) but Martial really seems to understand the system defensively, which we'll see how Weghorst takes to. His pressing stats are great and he's Dutch, and Bruno seems to be pressing well with energy more than pure nous, so that's promising.
Educated guess is it will come down to how much we're able to use his aerial prowess. So, it really might come down to the quality of Shaw, Malacia, Garnacho, Dalot, Wan-Bissaka, Rashford and Antony's crossing, and us winning good set pieces and lots of corners.
In any case, if we figure Martial is a 7/10 striker and we've just added another one, that's huge for the team and us finishing in the top 4 and being the CL next year and having it to offer potential signings, including ironically enough, a striker to replace Weghorst.
If this was a poll, I'd make it like this:
1 - Weghorst is a hit and we resign him for 2 years and defer signing a #9 until 2024 when Kane is available on a free or maybe someone else emerges the way Nunez, Haaland and Osimhen did before being bought for 60M+. Not sure I see that player on the market right now, now that Nkunku has gone to Chelsea.
2 - Weghorst outplays Martial and ends the season as our #9, but not by so much that we don't just let him go and replace him in 2024, or we surprisingly sell Martial and keep Weghorst as our 2nd striker.
3 - Weghorst is at Martial's level but Garnacho outplays them both and Rashford ends up our 9 in the season's biggest games (Barca 2nd leg, League Cup Final, the Liverpool, Newcastle, Chelsea and Spurs games).
4 - Weghorst is not as good a fit as Martial but scores or assists a few crucial goals off the bench and is a solid bench option, forming a nice tandem with Garnacho, with Rashford finishing games at RW a fair bit as they come on.
5 - Weghorst isn't very good but 1 crucial moment we all remember fondly to win the League Cup or the Europa or finish top 4
6 - Same as 5 but we miss out on top 4 and League and Europa so his contribution is forgotten.
7 - Weghorst is just kind of bad or gets injured
I'd say 3, 4, 5 and 6 are by far the likeliest. I'd bet on #4 at the moment. Very similar impact to Martial, just in a different way. End of the day, Garnacho is the best footballer of the 3 of them, and if he gets to come off the bench and run at tired RBs and RCBs then that should compensate for his youth.