Some players are more predisposed to injury than others. (Delph seems a bit that way inclined).
Over the past 3 seasons Mendy has missed 26%, 34% and 84% of league games though injury or suspension. His earlier ACL injury was not as severe, (and did not receive surgery), but a ruptured ACL injury is one of the worse an athlete can get. The potential weakness when turning, can create mental confidence/performance issues, even when the medical team give the "all clear" to play.
I want us to have top quality cover. I think Delph and Zinchenko have played well, particularly playing out of position, but IMO we are weaker with those options than with Walker on the RHS or a fully fit Mendy on the LHS. Maybe
@Dax777 is right and Zinchenko will provide a future solution? Time will tell.