I don't think Greenbull is coming to Wano to fight, but rather just to negotiate with the new shogun or recruit allies for whatever the hell has been happening outside of Wano. It would be peak government incompetence to send a single Admiral by himself to fight an entire enemy army that just murdered both Big Mom and Kaido simultaneously. Admirals are way too valuable to waste like that.
Plus, Yamato could probably defeat or draw an Admiral entirely on her own if need be, considering that she basically held Kaido (the single most powerful man on the planet up to this point) to a draw for 10-15 minutes with minimal injuries. Yamato is the strongest character in all of Wano right now after Luffy. It wouldn't make sense story wise for any Admiral to be that strong.
Plus, Yamato could probably defeat or draw an Admiral entirely on her own if need be, considering that she basically held Kaido (the single most powerful man on the planet up to this point) to a draw for 10-15 minutes with minimal injuries. Yamato is the strongest character in all of Wano right now after Luffy. It wouldn't make sense story wise for any Admiral to be that strong.