Matt Cassel started 6 out of 16 games in 2013 (7 games if you count the win against the Bear). MInnesota is a run-heavy team with the best running back in the NFL so Cassel naturally won't get a huge amount of attempts.
Greg Jennings' stats for 2013: 68 receptions, 804 yards, 4 touchdowns.
This is where it gets interesting. 59% of Jennings receptions, 61% of his yards, and 100% of his TD's, all came from Matt Cassel. That's remarkable considering Matt Cassel only started 37% of the games.
Now what if Matt Cassel was the starter from week one?
If we take Cassel's numbers and project them as if Cassel started all the games in 2013. His total stats would be 29 TD's, 24 INT's, and 4818 yards. All those would be career high numbers despite his successful break-out season in New England and Pro Bowl-season in Kansas City. The high number of turnovers is bad, but the projected yards would place him 3rd among QB's last year, the TD's would place him 6th.
Interceptions are often a result of poor chemistry between the QB and the receivers. Cassel is back as the starter and most likely with a greater chemistry. I think it would be fair to predict that the turnovers will go down this year.
Think how much better Greg Jennings' stats would have been if Cassel started from the beginning.
Fun fact: Greg Jennings played in all but one Vikings game in 2013. Guess you can say he also silenced the durability critics.