Got to take into consideration as well there is a reason 21 year old defenders don't tend to reach the finished article until way into their late 20s and particularly in England without a winter break its a nightmare on the legs and mentally.
Realistically at 21 during a busy christmas period you would have 3/4 options to keep the rotation, even varane last season at madrid was in and out of the side in place of pepe and thats in a league with a much less busy fixture schedule.
Then throw in FM who hasn't had a break for a very long time with the Argie league and copa libertadores and still adapting to life in a new culture and league, having to play every game due to injuries to browning and jags its left these 2 in the deep end and struggling to swim.
His passing in the last few games has been way off and its most likely because he is mentally drained, he has always defended in a wreckless lunging way thats not new but his poor passing suggests tiredness to me and we really need to add another option regardless of jags returning.