Wouldn't be defensively strong enough IMO. Locatelli would be more a Pogba replacement in that formation, one more suited to play in the midfield two but who would still need his partner to be defensively solid. In theory at least a Locatelli-Fred combo could be a similar combination as the Jorginho-Kante duo that has established themselves as Chelsea's first choice and helped them to the CL title.
I feel it would be defensively fine for like 2/3s of our games. We dominate most teams anyway. We played a EL final with Pogba in CM next to McT without problems (we dominated the midfield and didn't concede chances, bar the set pieces).
But yes for the tough games against strong EPL opposition or for some tricky away European ties, then Locatelli and Fred behind Bruno/Pogba would be the best option.
In any case, can't really plan a future with Pogba unless he commits by signing a new contract. Which doesn't look like it's happening, right now anyway.
EDIT: We should really try to hijack Locatelli's move to Juve, but it doesn't even look like we're targeting him atm.