I can't remember in football, but it definitely happened in baseball. And it worked, too. The Phillies made the world series in 1983 after firing their manager midseason while in first place at the time of firing. Their general manager managed the second half of the season and postseason. (
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It would be really easy to do this. They are actually well structured to make this change. Fire Garrett, give total control of the offense to Moore, give Marinelli back total control of the defense, and make Richard the HC. It can't possibly be worse at this point. What's been going on this season has been unacceptable.
And the best reason to do it is they could get a jump start on finding Garrett's replacement.