#2 Poor domestic record
Zidane excelled in the Champions League as a player, and it has been no different as a manager so far, as he became the first one to win three back to back Champions League titles.
With great achievements come caveats, and Zidane suffered from the same problem as his one time boss Carlo Ancelotti. He was unable to translate his European success to the domestic stage.
Although he won the league in his second season, his domestic record in the first and third seasons were less than impressive. He had a disappointing title defence last term, finishing 17 points behind eventual winners Barcelona.
While it has been exactly 10 years since United last won the CL, and you need to be at least in the final stages every year to be truly considered an elite club, it's the league title that United fans most crave for, especially now that their noisy neighbours won it at a canter last term.
Zidane falls short in this regard. Although he would guarantee European success at United, his managerial record suggests that he would likely not be able to bring that elusive PL trophy back to Old Trafford.