Complex sentence: It was well documented that Mansell and Ron Dennis never saw eye to eye but with McLaren's sponsors wanting a world champion, Dennis had only two options: the second option, Schumacher, was already taken, which left Mansell.
Split into simpler sentences: It was well documented that Mansell and Ron Dennis never saw eye to eye. Dennis also had two other options: the second option, Schumacher, was already taken, which left Mansell.