Tim Drevno has served as an assistant coach under Jim Harbaugh for most of the last 15 years, but that prolonged faith is reportedly the root of some frustration at Michigan.

According to Gregg Henson of Gridiron Now, sources at the school said nobody in the athletic building can understand how offensive coordinator and line coach Tim Drevno remains employed.

Henson cited the departure of assistant coach Greg Frey, who left Indiana for Michigan in 2017 but joined Willie Taggart’s staff at Florida State this offseason. Frey is a respected O-line coach, but Drevno apparently kept control of the unit.

And the offensive line has been a major problem for the Wolverines since Harbaugh’s arrival. Michigan’s total numbers don’t look terrible, but the running game has consistently failed to reach three yards per carry against ranked opponents, such as Ohio State, Michigan State and Wisconsin.