#3. Lack of available options:
Despite rumours of Jose Mourinho may get sacked, it's safe to say there are not many managers available and good enough to replace him. Zinedine Zidane is the man United are linked to but his agent clearly stated that he won't manage teams until next season.
Another viable option is Antonio Conte who clearly had a sketchy last season with Chelsea at English Premier League although both these managers would suffer without proper players to implement their tactics.
Even with poor form and results, it would not be wise to fire Mourinho midway through the season. Their latest comeback against Newcastle United might just unite the team now more than ever and showed us a flicker of hope which is enough to turn things around.