I feel like Lampard gets too easily fixated on formations once they work. 4-2-3-1 was never the right choice today, but it's what we used almost certainly because we beat Barnsley with it mid-week. West Brom play 3-4-3 - the vulnerable spot there is behind the wingbacks; either you get runners past them or isolate them 1v1 to then look for cutbacks to the halfspace as the CBs drop. Very obviously the best formation to use here is a 4-3-3, Werner LW, CHO RW, Mount and Havertz as 8s, Kante behind. Instead we used Mount at right attacking mid where he was atrocious & killed so many attacks, since he's not great at running in behind and he's certainly not a 1v1 dribbler. Our front four all wanted to play centrally right into the teeth of 3 CBs and 2 CMs, so our only width was from our fullbacks who thus left tons of space behind ostensibly to be covered by the pairing of Kante and Kovacic, who just don't work as a pivot.
Frank deserves credit for switching to 4-3-3 for the second half because this fixed literally every single issue we had, but then he switched away from it for some reason which was insane. I don't know why we aren't just playing 4-3-3 - we played our best football last year after the restart in 4-3-3, Havertz and Mount both work as 8s, and we can have width in our attack without having to use our fullbacks.
Regardless, it's far too soon to consider sacking him obviously. We need the whole team fit; Ziyech and Pulisic are huge misses right now.