This move will be disappointing to some fans, but it was not completely unexpected. The undrafted rookie free agent running back had a good summer, but he got caught up in a numbers game on the Eagles' depth chart. Mostert was behind DeMarco Murray, Ryan Mathews, Darren Sproles, and possibly Kenjon Barner as well.
Speaking of Barner, this seems to bode well for his chances of making the final roster. Earlier today it was reported that Barner was made available for trade. That still might be true if the Eagles only want to keep three running backs, but one could argue Barner has earned his roster spot on the Eagles.
Mostert is still practice squad eligible and could very well clear waivers and make it to the practice squad. Only time will tell in that regard.
UPDATE: This seems like a positive sign:
We are so thankful for the opportunity the Philadelphia Eagles gave my client Raheem Mostert. If he clears waivers we'll definitely be back!— Brett Tessler (@TesslerSports) September 4, 2015