Depressed Dog

[lucyB]

Hi..
My 6 1/2 yr old 21 lb Scottish Terrier has been diagnosed w/ a torn cruciate in his right rear leg which he pulled a little over 2 weeks ago chasing after the ball that he is totally obsessed with chasing. Because he is often walking on this leg we have decided to go the non-surgical route with our fingers crossed. He is still a "tripod" when it comes to anything more than a slow walk at this time.
He is on Deramaxx and Glucosamine. My question is can dogs become depressed?? He has been very quiet and in his bed most of the time.
My usually active dog is just so quiet. He comes to us with ball in mouth in hopes that we will play with him but of course we cannot play at this time.
He has never been a "chewer" so I do not think even a chew toy will keep him occupied.
Any suggestions??