The Patriots have defined roles for their running backs. When they want a physical presence with the run game, Stevan Ridley comes in. Bring in the tight ends, run the zone blocking scheme, let him stretch the defense and make one cut and run downhill. He's the best they have to do that.
Danny Woodhead's role is the sub back. Runs out of shotgun. Draws. Not the workhorse back, but a complementary back when they are in their spread packages. That's what they were majoring in last night.
No reason to get alarmed and drop Ridley for Woodhead. As always, the Pats game plan is never easy to figure out, but this one makes sense as to why Woodhead got more work. Ridley is still their best running option in the formation they like the best - two TEs.