Justin Maxwell broke his hand os in the 15-day D.L. and may be out longer, Grossman has shown no power yet at AAA but is getting on base at a high rate. Grossman may not get much PT but I suspect with the Astros going nowhere Porter may play him some. From a fantasy perspective he might help with a few steals. On a good team he profiles as a 4th outfielder as someone who can play D and post a decent to good OBP, but the Astros are not a good team, so who knows if he might get more of a chance now.
For fantasy players who can't hold on to prospects for a long time because of league rules, Grossman is your classic low-ceiling but high-opportunity prospect -- we can all name 100 prospects who excite us more but most won't play in the Bigs this year.