That will probably be over 40 million for next year. Even though you pay your rookie QB very low it is tough to pay a backup that money.
Unless you tell McCarthy he's not going to be the starter.
A few things to think about.
First, McCarthy had to have a follow-up procedure mid November due to swelling in his knee. He won't be cleared until spring at the earliest. I'm not sure I would be ready to roll the dice that 1) he will be cleared and 2) he's ready to be the starter.
Second, Darnold has led the Vikings to a playoff spot, a potential #1 seed and currently best record in the NFC. While $40 million is a lot of coin, the current other options on the roster are Daniel Jones and Nick Mullens. Yikes. Give me Darnold all day versus those two.
Third, Vikings will have around 77 million in cap space. Jefferson is locked up for a few more years so they can afford to spend that money for one year.
It really comes down to going with the devil you know versus the unknown quantity. Darnold may be a one hit wonder but he's looked damn good for most of this season.
Personally, I would go into next season with Darnold the starter and let McCarthy learn. If Darnold turns out to be a dud, give the reins to McCarthy.