Well by total offense UF was ranked:
2005: 59th with Mullen
2006: 27th with Mullen
2007: 15th with Mullen
2008: 21st with Mullen, but Mullen leaves after this season
2009: 10th without Mullen, best Meyer's offense at UF, then Meyer tries to quit.
2010: 85th in Meyer's past, best if used by date, season
All rankings from:
https://www.teamrankings.com/college-football/stat/yards-per-game
So if by "different the minute Mullen left," you meant Meyer had the best offense during his UF tenure, then I see your point but I do not see that 10th is that much different than 15th and some of the other seasons. If you meant that, UF got worse "the minute Mullen left," then you are factually wrong.
To me the numbers say UF only got worse during the Meyer halfhearted season. Of course UF stayed worse as UF hire Muschamp and then McElwain, but that was to be expected and a different subject.