That is a theory. We don't know it. As a starting back in college, I bet he has experience in pass blocking. The likelihood is Tomlin is a little slow to promote talent and it often takes an injury to find out the guy replacing him is better than the guy he thinks should start.
To use a recent and impactful example, many including myself wanted M. Bryant to get a hat. Tomlin was tardy, but once he id Bryant added a lot to the offense. I'm not saying McFarland will be a pro bowl level back, but I am saying Conner is broken down and slow, and Snell quickly lost his luster with that fumble.
Randy Fichtner's run, run, pass, or draw on 2nd and long offensive play calls are not working with Conner or Snell, they are too slow to hit the hole and can't run outside. Defenses know this and bottle them up. But with McFarland, he hits the hole much quicker and can take it outside to beat a player to the sticks.
Suddenly the offense is harder to defend. Like I've been saying for years, Tomlin holds us back. He picks the starters. I think Chucks by the way is a way better pass blocker than Banner was and should have started over him. We'll see if Tomlin misjudged that one.