Why the Lions didn't have anyone waiting in the wings to take over as OC after Ben Johnson left

That's a really good question. The Lions had all the time in the world to build a guy up, and they knew that Johnson was going to leave at some point? So why didn't they have a guy waiting in the wings? Here's the thing: they did. They had multiple guys whom they were building. The problem is that all those guys are left with Ben Johnson and Aaron Glenn.

Tanner Engstrand

Engstrand was long considered to be the successor to Ben Johnson. A lot of people thought for well over a year that he was going to be the Lions' OC after Johnson left, and then when it came to the time to do that, there was a really different feeling in the air. Suddenly, it no longer felt like it was going to happen. Maybe the Lions didn't feel he was ready yet, but Aaron Glenn did. He took him to New York to be his offensive coordinator.

J.T. Barrett

Barrett was another guy who was a lot bigger part of the Lions offense than you might think. Ben Johnson gave him a ton of credit for helping come up with and install a lot of the specials the Lions ran when Johnson was there. Johnson took him with him to the Bears to be his quarterbacks coach.

Antwaan Randle-El

Another guy who absolutely is ready for an OC job and was ready for it last year, too. Again, Ben Johnson took him with him to be his assistant head coach and wide receivers coach.

There are still some guys left in Detroit to keep an eye on

In addition to Barrett, the other guys who helped with specials were current Lions assistant offensive line coach Steve Oliver and assistant tight ends coach Seth Ryan. Both of these guys are growing in the NFL, and while they're probably not ready to be offensive coordinators just yet, they could be down the road.

So that's it, the three guys who could have been considered all left. Along with them went any chance the Lions had of a backup plan.