Brock Purdy is one heck of a quarterback, he’s only special when Christian McCaffrey plays.

In Purdy’s NFL career, he’s 18-6 when Christian McCaffrey is in and 5-7 when McCaffrey is out. That’s a big difference. Something about McCaffrey’s skill set makes Purdy perform at his absolute best.

And McCaffrey isn’t the 49ers’ only talented weapon. They have the best tight end in the NFL — George Kittle. They have a terrific wide receiver — Jauan Jennings. Plus they always have an efficient running game no matter who the running back is. Last season, they averaged 4.7 yards per carry — seventh-best in the NFL. And McCaffrey hardly played.

But what they don’t have when McCaffrey is out is a player who can run routes and catch passes out of the backfield. That ability seems to unlock the 49ers’ offensive scheme and Purdy’s full capabilities.

Purdy doesn’t quite have the arm to threaten defenses deep and outside the numbers the way Matthew Stafford does. So Purdy wouldn’t necessarily take full advantage of a true No. 1 wide receiver such as Justin Jefferson or Ja’Marr Chase.

That’s why Purdy needs an elite check-down option such as McCaffrey. With that player, Purdy doesn’t try to do too much. Without that player, Purdy tends to hold the ball too long, force it downfield and get intercepted with the game on the line.

And once defenses sell out to stop Purdy from passing to McCaffrey, the rest of the passing game opens up, especially downfield.

Unless the 49ers are confident they can find another McCaffrey in the upcoming draft, they should think long and hard about paying Brock Purdy more than $40 million per season. Because he’s only worth that much when he’s combined with McCaffrey.