Tomori, Mount and Wilson were only loan signings and considering Derby were able to make 8 signings in that summer that Lampard left them, then isn't it their issue that they couldn't find anyone decent to do a good job for them? Plus, they hired Phillip Cocu, who has more experiences as a manager than Lampard, to replace him and yet, he couldn't quite manage a play-off place finish.
Also, we should know better than most that just because good signings are made, it doesn't necessarily they will be good unless the manager knows how to make good use of them at least. Lampard certainly did that with Mount, Wilson etc in that season and mind you, Wilson hasn't come close to replicating the numbers he had at Derby since then.