Based on what? Top 5 in the league last season shows otherwise. But don't let stats get in the way of your thinking.
Oh jeez guy. I didn't want to be an ass. I didn't want to be.
OK, here we go. Top 5 in what, exactly?
QB Passing Yards? He was a decent but not glorious 12th in the league.
Perhaps TD's, right? No, he was 12th in the league.
OK, gotta be QB rating, right? Nope, admirable but 8th in the league.
All in a career year playing for a creative offense with fantastic weapons (Diggs, Thielen, Rudolph might be the best receiving punch in the NFL). He had a world class defense behind him. That's a Super Bowl contending car giving a driver the keys and saying "Don't wreck, fella." To his credit, he managed the game. This is his absolute peak performance. You want to pay him based on that and gamble we can recreate it on this 0-16 team?
I think you're more likely to see the Case Keenum whose Passer Rating was in the 70's 3 out of his other 4 seasons in the NFL.
But by all means. Don't let reading, comprehending, or counting get in your way of thinking.