First of all, I can't believe some of you are actually concerned and think that he really was so badly injured that he'll have to miss the rest of the year.
This is SOP in college football... you try out a guy that you think may contribute his freshman year, and if he's not quite "there" or there's another backup that can do his job, the freshman in question conveniently gets "injured" for the rest of the season.
And it's granted almost everytime because all the NCAA requires is a doctor to sign a document saying that the player is injured... no one is going to come and physically check Gardner to see if he's injured, and no one will care that he was taking practice snaps two weeks ago. Hell, Devin will probably still be practicing at 100% for the rest of the year.
For another example of this, see Nick Hill at MSU... he too played early but after the emergence of Le'Veon Bell, Hill got "injured" and will redshirt.