The first time it happens it's on them, the fifth time it's on the manager for not doing anything about it. Players kept making mistakes and suffered no consequences for it. That was the problem with Ole - both regarding his staff and players - he accepted too many mistakes and sub-par performances. We've become excuses United with Maguire as captain sorry of a squad accustomed to making excuses. That is not how you win trophies.
I don't blame Ole - his job was clear. Get the squad happy, recruit well and start making players believe in the Man Utd ethos. That job was done last season. The right thing to do, from a footballing development perspective would be to thank Ole for his work this summer, let him leave as a success and bring in someone tactically exceptional who would take us to the next level. We didn't do that, and he left in a cloud of failure instead. Which he didn't deserve. All because of a complete lack of succession planning. The entire point of having a professional footballing department or board of business people in charge is to make these decisions and plan ahead - fans are too emotionally invested.
The players haven't been good enough, but they have suffered no consequences, that is on the manager. However, the manager was hired to lift the toxic cloud Mourinho put on the squad, make players happy and prepare the team for success. That job was all but done this summer and would have been a natural point for the leadership - or lack thereof - in the club to act. Ole was out of contract next summer (so mutual termination was within the realm of possibility) and Nagelsmann, Rose, Pochettino, Ten Hag, Conte, all of them were gettable this summer, instead they acted on sentiment extended the contract and put us in the situation we are in where we have to pay to relieve a manager who just signed an extension of his duties and bring in a new man, possibly pay for both an interim and a new manager/head coach. Ultimately it's their fault.