He's gotta be the 5th sub in that case, even if we're chasing several goals. Gray must start and if we're are goal away (in any of the games) from relegation in the second half then Simms comes on.
Next I'd put Townsend on ahead of Maupay and shift McNeil into number 10 with Gray and Simms up front, then bring on Onana (expecting Gueye, Garner and Doucoure to start) up there in a Fellaini-like role, and after that Keane in a free role to just get his head on the end of long balls.
If all else fails, shift Onana and Keane into central number 10 roles and smash the ball up and have Maupay, Gray and Simms ahead of them looking to fetch the knock on.
The game may end like this:
Pickford
Holgate-Mina-Tarkowski
Gueye
Onana-Keane
Gray-Maupay-Simms-McNeil