That's mainly down to tactics and instructions by the coach though, just look at the distance covered from our players when Mourinho was in charged vs Solskjaer's now and you will see why
There is a long thread Workrate,
https://www.redcafe.net/threads/workrate.421867/page-22 which examines this issue. I have posted various individual and team distance covered stats in that thread, during both Mourinho's and Solskjaer's time in charge, including team distance covered stats for all our CL matches this season. The official UEFA CL stats are here:
https://www.uefa.com/uefachampionsleague/season=2019/statistics/round=2000980/players/index.html
To the extent that it is down to the coach, that is merely a case of what distance they consider acceptable, rather than a maximum figure. For instance, Valencia ran about 9.0 km per 90 mins last season, which Mourinho was prepared to tolerate. Our other right-backs ran considerably further. Dalot ran 10.7 km against Young Boys this season. Similarly, Martial and Rashford covered very different distances when playing left wing for Mourinho (the precise figures are in the Workrate thread).
Under Mourinho, Matic, Fellaini, Herrera and McTominay all averaged well above 11.0 km per 90 mins, typically closer to 11.5 km per 90 mins. Solsjkaer wants his team to cover more distance. Ndombele would have to find an extra 2 km per 90 mins. Is he even capable of that ? Can he do it on a regular basis ?
If it wasn't a huge warning sign, I wouldn't have posted the figures in this thread in the first place.