The Vikings traded down from the eighth overall pick to the ninth on the opening night of the Draft yesterday before choosing linebacker Anthony Barr despite Manziel still being available.
They did eventually satisfy their need for a quarterback, trading up to draft Teddy Bridgewater with the 32nd overall pick, and Jennings believes that they made the right decision to ignore Manziel due to the 21-year-old's personality.
"I truly believe that given any player, if they're willing to work and willing to be better, that [head coach Mike Zimmer and offensive coordinator Norv Turner] - two, literally, head coaches that we have on both sides of the ball - they can make anybody better," Jennings told reporters.
"I just didn't think - which was probably what their thought was - that Manziel fit what we were trying to get done, personality-wise. When you're making a coaching change, and you're raising the bar of accountability and you look at the past history - it's tough, because you never want to hold anybody to that.
"You always want to continue to give somebody the benefit of the doubt. But when you're investing the amount of money these owners and these clubs are investing, that carries a lot of weight."
Manziel was eventually drafted by the Cleveland Browns with the 22nd overall pick, although reports have suggested that the Vikings were keen to trade up in order to select the former Texas A&M Aggies quarterback themselves.