Montgomery has a lot to prove, but the dude WAS a running back. He kept trying to keep his weight down in order to play WR. He is a special type of athlete that has the ability to play both RB and WR. There aren't many of those guys that have played both in the modern era or otherwise. As for his ankle, the guy Chargers defender landed right on top of it in 2015. He suffered a high ankle sprain and was quoted in 2016 as saying the following:
http://www.espn.com/blog/green-bay-...-montgomery-reveals-severity-of-ankle-surgery
I broke my right ankle twice in HS football, never missing a game at center and NT. In my early 20s, I got a high ankle sprain playing ultimate frisbee when I jumped up and landed on top of someone else's foot, causing my right ankle to roll badly. I was in a cast for two months, hobbled for a month or two after that, and wear an ankle brace to this day when I play competitive sports. Montgomery had one nasty injury and kept trying to come back from it. He learned his lesson and made through 2016 healthy.
He has a lot to prove but I don't think it's on the injury front. He needs to prove that he can pick up the blitz, run the ball between the tackles, but most importantly, he needs to show that he came replicate a James White type of back who can kill defenses by catching the ball out of the backfield either by design or when Rodgers needs an outlet.