Mariota's main problem was injuries.
He showed right away in his rookie season that he's an NFL starter.
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19 TDs vs 10 INTs.
The Titans D ranked 27th in points allowed.
The starting RB that got the majority of the carries was a no-name UDFA averaging 3.6 ypc.
The Titans won 3 games with Mariota; and none without him.
Don't forget that in his last season with the Titans, his TD/INT ratio was 7/2