Nick - Keep the pup calm & contained(Crate) and gewt it to a Vet. The 1st order of business is an evaulation. It could be anything from a bad sprain to a tear. My Rottweiler did something in her kennel run one day (probably upset at the UPS driver) she was limping when I got home. We weree lucky the Vet felt it was a very slight tear if any at all. This is a dog that runs 5 mi with me on my Mtn Bike. For rehab the 1st 2 week I keep her crated if she wasn't on lead. No walking or play. Then the rest of the 1st month loose in the house no kennel time. 2nd month we started progressively increasing our walks. The 3rd month we strated to swim again. I had to wait as she gets to fired up at the beach. I didn't want to let her reinjure herself BEFORE we even got in the lake. Then in the 3rd month we got back to the bike. I was real happy with our results. I have a friend that her Rotty tore it's ACL...it had two operations and never treally got better. She refused to take her time rehabing(reason for 2nd surgery). It's not easy keeping an active adult dog calm for 90 days....It will be harder with a pup. I do feel in the long run you'll be happy if you force the pup to take it slow. Best of luck