Tim, I experienced very similar. Onyx was 4 when while earning his last leg for his Sr title was cruising through the test... you know when a dog has the game figured out.... he would heel to the line, know where to set up and look for each of the marks.... he was on pinned the marks, did the land blind like he had planted the bird... on the last series, he comes out of his crate with a gimp. I had some frozen waterbottles in his crate as it was hot, figured maybe he was laying on it and it was numb. I worked him out so that the limp was not noticed by the judges... as we just had the water series left. He made short work of the marks and blinds too, took just 2 whistles on the blind. I was so proud. Next day, still lame... a few days later we go tot he vet. Onyx is so muscled he can't feel anything, so we have to sedate to determine... even then he is not confident, so he sends me to an ortho guy... he agrees, that he will need to sedate, and likely inspect to know for sure. Sure enough Onyx tore the ACL. Lost him for the entire duck season. From then on, I never let my dogs jump up into the truck. I built a ramp and used it on all the dogs ever since. So the following August, we are giving a retriever demonstration, and I have Onyx off lead. We approach the truck and he automatically sits. As I put away his collar, and leads, and reach for the ramp, I get into a converstation, and inadvertantly said OK. With that Onyx, leaps into the bed, but falls short, and catches the down tailgate at his ribs. Rolls to the ground and is now lame on the other leg! Sure enough tore the ACL too. The ortho guy suggested that he sees a fair number of ACLs on working retrievers, especailly those that are well muscled and have tight "perfect" hips. He thinks that with no relaxation in the hips, and strong muscle, that the ACL becomes the weak link....
My advice, get them fix, and get him in the water.... I unfortunately did not have the ability as the water is cold or hard in VT in the fall/winter... but as soon as he is allowed to get wet, walk him on lead into the water until he can not touch bottom, and them work him swimming only. Don't let him run in and out of the water, never touching bottom.... best rehab you can do.
Sorry for you... good luck. What is the ACL cost these days? I paid $1200 each time in the mid-90s