5 month old heeler, ACL tear?

Could be a ligament injury.  Could be a cracked bone.  Could be a pinched nerve. 

If he is still limping in the morning, I would get him into the vet ASAP.  Only a vet can tell you how serious the problem is and what treatment is necessary. 

with a puppy not bearing weight, I'd have been at the vet today.  If he's still lame tomorrow, definitely see the vet. Leg injuries are nothing to mess around with, especially in a pup.

could be something with the tibia bone too. those are very fragile in young pups

 don't wait - get him in. leg injuries left long can do much more harm than good. he'll, at the very least, need crate rest - no running until he's seem!

Thanks for all of the helpful information.  The Outlaw Josey Wales saw the vet today, and it turns out he broke his tibia bone in his back leg.  He is resting now on pain medication and will be having surgery tomorrow to repair the break.  The Doc said it will require two pins. 

I wish him a speedy and full recovery!

Fingers crossed for an easy repair, & smooth recovery. 

Just added his picture, he is a sweet and devoted little pup, but full of energy and puppy clumsiness.  It's hard to see him struggling so.  Thanks again.

poor kid - fortunately - it's a VERY common puppy injury. I've got one at work now that's going to be going in for surgery soon now. he jumped off a couch, landed the wrong way.

speedy recovery pup!

onebear

it turns out he broke his tibia bone in his back leg. 

Here is a description of that injury and its treatment:  http://www.petplace.com/dogs/tibial-tuberosity-avulsion-fractures-in-dogs/page1.aspx

"Tibial Tuberosity Avulsion Fractures in Dogs

An avulsion fracture occurs when a bone breaks and a fragment of the bone is separated by the pull of an attaching muscle, tendon or ligament. The tibial tuberosity is the prominent bump on the front and top of the tibia, the shin-bone, below the knee joint. This tuberosity attaches the patella (knee-cap) via a strong thick tendon of the quadriceps muscle group. A fracture of the tibial tuberosity often results in an avulsion fracture, by virtue of the pull of the quadripceps muscles.

This fracture tends to occur in younger dogs, before this area of the tibia has fully grown and fused to the rest of the bone. The fracture occurs in puppies in association with trauma, often falling from a height and landing with the knee in flexion, tearing the bone fragment from its normal position."

Hope the surgery goes well!!