To put Mullen's MSU record of 2-15 mark (?final ranking?) against AP teams in perspective, here was Mississippi St's records before he took over the job in 2009:
2006: 3-9 (1-7)
2007: 8-5 (4-4)
2008: 4-8 (2-6)
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So basically, the Mississippi State job was a rebuilding job. Mullen went there to try to change the culture to a more consistent, winning culture where it did not exist before since the Jacki Sherrill days of the 1990s.
That is what the Florida job has also become - a rebuilding job. Florida had two losing seasons in the last five years ('13-'17: 4-8, 4-7) when previously Florida had winning seasons from 1980-2012 (33 straight winning seasons!).
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Here is MSU's records in the 9 years that Dan Mullen was there:
2009: 5-7 (3-5)
2010: 9-4 (4-4) <-- 3rd best SEC-W record
2011: 7-6 (2-6)
------ Addition of Texas A&M, Missouri to the SEC --
2012: 8-5 (4-4)
2013: 7-6 (3-5)
2014: 10-3 (6-2) <-- 2nd in SEC-W
2015: 9-4 (4-4) <-- 4th
2016: 6-7 (3-5)
2017: 8-4 (4-4) <-- 3rd in SEC-W
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With national/SEC championship powerhouses in the SEC-W (i.e Alabama - '09, '12, '14 '15, '16 Auburn - '10,'13,'17 LSU - '11), Mullen did not or could not win the mighty, competitive SEC-W division. There was also the Johnny Manziel Texas A&M teams to contend with along with a resurgent Ole Miss program (which will soon get hammered by the NCAA for recruiting violations).
However, what Mullen had ultimately achieved at Miss. St was getting the Bulldogs to have winning seasons in 7 out of 9 seasons. When previously there were losing seasons, MSU now averages 7-8-9 wins a season. That is a remarkable turnaround at Mississippi State.
Every two or three seasons, MSU had strong enough teams that could contend for the SEC-W title. Mullen is probably walking away from another good team at Mississippi St to take on the Florida job!
At Florida, Mullen will have the Gators contending for the SEC-E title almost every season. The Gators just have to find a consistent quarterback which has been THE story for the last 7-8 years.