Unfortunately injuries are part of the game but Montgomery's size is fine to be a durable running back in the NFL.
He runs pretty high like Starks. Some runners need to do that for vision and quickness. You can get away with that if you get low before contact. However, over the course of 250 carries a high runner is going to mistime the hit and absorb more than his share of low hits to the legs instead of contacting at the pads than a runner who stays low.
The good ones, like Lacy, can stay low from the get go and still see the hole developing, with the quickness at the line (yes, Lacy was quick in 2015) to redirect.
The fact Lacy has had a 4.4 career average (and especially that 5.1 last year before going down) without the occasional long run to pad the average, is pretty remarkable. Why he's been less effective in short yardage/goal line seems to be a function of him being a finesse back at the line and a power back when up to speed at the second level. That's why you have a guy like Ripkowski.
Of course, being a low pad runner is hardly a guarantee. Few backs get through 16 games without missing time, and careers tend to be short. But the higher you run, the bigger the risk.
Montgomery is pretty amazing in one respect: He
wants to play the position when the prospectus says he could have a longer career as a slot receiver. And he surely knows second contracts for RBs have a ton of risk aversion built in unless you're a Peterson or McCoy or similar high production, durable back.
Maybe Montgomery thinks he can be the second coming of one of those guys. God bless him for the effort.