It's clear fitness is a big issue. Partly down to training and conditions but partly down to the type of players also.
Lukaku was great for the first half an hour but he honestly looked like he couldn't breath at one point. His body just isn't designed for a high intensity game. (Unless he's on Peps special juice). Same with Matic and Mata.
This isn't a post against these players as I actually like all 3 of them. Just an observation that I don't think they can play high pressing, high intensity football for long periods of time. Look at the body shapes of Bernado silva, Aguero, Mane, Salah, Keita, Fernandinho, Sterling and Kante etc. They are built like long distance runners. There will always be exceptions to this, but as a rule if you want to play that style, footballing ability is only 50% or less of the battle and physical attributes are massively important.
Is Wijnaldum better at football than Matic? No.
Is Henderson better at football then Pogba? No
Is Mahrez better at football than Mata? No
They have legs though and that's what matters if you want to play that style. At the moment we only have Rashford, Herrera, Lingard and Mctom as the midfielders and attackers that are able to cope with I from a physical point of view.
From a distance covered point of view the likes of Mata and Matic are more than comparable with their Liverpool/City counterparts (CL stats per 90m: Mata 14.2km vs Salah 11.1km vs Bernardo 12.8km / Matic 12.7km vs Henderson 12.8km vs Fernandinho 11.8km). Mata's are a little skewed due to sub appearances but his work ethic shouldn't be a question.
However the same cannot be said of Martial, Lukaku and Pogba; who all cover less distance than their counterparts (CL stats per 90m: Martial 9.9km vs Mane 11.8km vs Sterling 11.4km / Lukaku 10km vs Firmino 12.7km vs Aguero 10.8km / Pogba 11.7km vs Milner 13.5km vs De Bruyne 13.1km.
It's worth noting however that all these averages from United across the board this season have jumped staggeringly under Solskjaer:
Martial CL average on 17th December - 8.9km
Martial average today - 9.9km
Lukaku CL average on 17th December - 9.1km
Lukaku average today - 10km
Pogba CL average on 17th December - 10.5km
Pogba CL average today - 11.7km
Lingard CL average on 17th December - 11.7km
Lingard CL average today - 13.1km
Fred CL average on 17th December - 11.8klm
Fred CL average today - 13.1km
Young CL average on 17th December - 9.8km
Young CL average today - 10.9km
Given that the previous averages were based on 6 CL games and the new averages are based on 10 CL games then to improve these stats by this margin (c. 10%) has taken a huge improvement.
This shows the work that Solskjaer is doing but it's going to take far more than 4 months to get us where we need to be.
Seperate to this though we also have players who struggle to properly close the opposition down aggressively. Covering distance is great but if you stop when you're 3 yards away from the opposition, rather than put in an actual tackle then the distance covered isn't that helpful when you aren't in possession. This imbalance is down to squad composition. The likes of Matic, Pogba, Lukaku, Mata and Martial don't actually put in tackles or chase down opposition players with intent. They all give our opponents space to pick out a pass, which hurts us regularly.
Against the top teams that play our from the back the main way to counter them is to press their defenders. Liverpool have beaten pretty much every top team in Europe by adopting this approach. However it requires pretty much every outfield player outside of your centre backs to aggressively press. Watching us under Ole I believe you can tell he wants us to do this, but it's impossible with only 6 players who fit this profile in Herrera, Fred, McT, Lingard, Rashford, Shaw. Young/Sanchez try but their legs have gone (and most of the others can't do it for 90 mins... yet).
If this is our style which I believe it should be then we obviously have some huge questions as we have 5 players who are all very frequent fixtures in our XI (who we paid £300m out for) who aren't compatible with this style. Not only that but it's a style that has a prerequisite of all players following as if one doesn't then it creates a simply opportunity for the opponents to play through the lines.