I don't think injury is the problem with keita. Keita can do stuff that no one in pool midfield can do except thiago now but it is imperative for that midfield to press and win ball and he can't do that.
Ndombele last year pretty much didn't cover enough ground to play for Mou. Mou pretty much criticized him publicly so many times.
"Moreover, some of the players you indirectly mentioned are his signings and given Roman paid a lot for them, then Lampard is trying to play them together"
This is a problem. We have played, i don't know over 20 games now, you know what you will get from each player till the end of the season, it is pretty clear.
Right now lots of people is criticizing lamp for not utilizing his player properly especially Werner . IMO, what he is doing right now is forcing Werner Pulisic ziyech and Havertz to work. Will it eventually work? Maybe if they get better but right now it is clear how it is not working. Just pick combination that clearly works and bench the other, you can always rotate.
Btw before people will say with better manager it will work, hmm, take griezman, Atletico star man, France star man, a fantastic player but it was freaking obvious he won't work in Barca, two seasons, 3 different managers, still not working.