Solskjaer needs to take some blame for depth issues.
I feel De Beek not really acclimatising to the squad is actually on Solskjaer. It took far too long to give him his first Premier League game. He didn't get any real time until the 9th game of the season, despite us actually being very poor during that period. He proceeded to only see 137 minutes in the Premier League in the next 15 games before he was injured on the 21st February 2021. He also only saw 199 minutes in the remaining 9 games after he came back from injury.
Out of 3,420 minutes over the season, he saw a total of 513 minutes, most of which were sub appearances spread out in small numbers. And 183 of those minutes were only seen after we secured top two. Are you really thinking that De Beek wasn't good enough to start against Burnley, West Brom, Sheffield Utd etc? He could have given players a rest, but Solskjaer refused to use the depth available, so they neither acclimatised nor reached match-fit levels.
Even Brandon Williams was frozen out of the squad this season. He may not be the best player, but it seemed very sudden from giving him games last season to giving him barely any in this one. He saw 187 minutes over the entire season. I'm sure there must have been one time before securing second place that he could have had some game time to get him ready for some games. In the end, he would have had no match fitness.
There are also times where he insisted on not using the youth players etc. even when the first team wasn't needed. Remember the second leg against Real Sociedad? Why was our first team needed when we trounced them 4v0 in the first leg? It was a perfect time to rest players, but he didn't take it. There is no point of depth if you are not going to use it.
Also, you have to ask: has Solskjaer made some players worse? I remember he got a lot of credit for Martial's improvement last season, but, if that was the case, what does that mean this season when Martial was rubbish. Does Solskjaer also take flack for him dropping off so far from last season? If you give one, you probably should give the other.