Considering this is his first week the entire year where he actually got some playing time, and he turned it into 158 yards on the ground : He deserves it.
Why should he win over Matthews? Matthews did have 120 yards, it's not like he had 3 TDs on 55 yds. Joe had a higher YPC by .3, 3 TDs(none of them goalline carries) trumps that difference in YPC.
Ryan Mathews is like the anti God. Rather than work 6 days and rest on the last day, Sunday, he rested for 15 straight weeks before deciding to play on the final Sunday.