The dressing room atmosphere plays a big impact on performances, that has always been a known thing. Ralf didn't help because he brought in MLS level coaches and the whole Lingard thing on deadline day and then never using him. Generally though I think he had an impossible task anyway, but he didn't help.
Fringe players being disgruntled means your training sessions each and every day are impacted. Players who are friends with others get impacted by it and start taking sides. You get constant distractions. You aren't training or working with a United group, but it's a group where half the people are putting in half the effort as they don't want to be there, and then it drags the quality of everything down. The morale was absolutely shattered last season, and that stemmed from a mixture of garbage coaching and garbage squad/man management. That's it. Henderson and Lingard are 2 of the biggest examples of garbage management. Both should have been sold ages ago if they wanted regular football, yet they were kept around to be "squad players", and then never used