A Rough Sunday for Walker Evans

Sunday was supposed to be a big day for Walker Evans; it was the 10th anniversary of Val adopting him and bringing him home, and it is basically the day we celebrate his birthday, and he turned age 12.
We set out to the Lasalle Park dog park as we do every weekend, and he was having a good time, trotting around and playing with other dogs. But Walker decided that he would start roughhousing with some of the larger dogs (Walker is 80 pounds), and one trampled him a couple of times, and Walker let out a scary whine and painful howl.
I went running and it was obvious that Walker had been hurt in his back hips/haunches, where he has arthritis. At first, he looked badly hurt and couldn't stand, but after a minute or so, he stood, then started walking and his limp was all but gone. We immediately went home.
Walker has been favoring his hips/haunches since we got home; regular walking has been OK, if a little slow, but stairs have been difficult for him, and he has been hesitating or not climbing them. I had to bring dinner upstairs last night, but he will have to go downstairs to go outside before he eats this morning.
Let's see what happens; he is a great dog.