For the Broncos, here’s the job description of the middle linebacker:
Play from C-gap to C-gap in the running game, while flowing to the football, and mostly being unblocked.
Play mostly man-to-man coverage on running backs and tight ends in base passing situations.
Play some zone coverage, at times.
Come off the field in sub package situations.
That's not the job description for every MLB in the NFL, but it's what the Broncos need from the position. It doesn't matter if a guy wouldn't fit everywhere else; only what he can do for Denver matters. I am highly confident that Nate Irving can do those few specialized things pretty well. He’ll give up a completion here and there on first down, but every team is less effective defending the pass on first down than they are in obvious passing situations.