Hodie is right, there's not really enough info here but if it's a back leg, it sounds like a cruciate ligament to me too.
Been there, done that twice recently. It all depends on how bad it is.
If it's torn, it may need surgery and the best time to do that is right now. Waiting won't do any good and they can't fix it if the injury is too old according to my ortho vet. Don't expect the dog to ever really be back to 100% pre-injury and be aware, the chance of it happening again is high.
If it's only strained, crate rest, no running, short leash walks to go potty only. Joint supplements help. Rehab by manually manipulating the joint and the entire leg may be suggested. Expect to keep exercise restricted for at least 6 weeks gradually building back up to normal after a few weeks.
If I were you, I'd be back at the vet today. The vet can sedate him and do a simple "drawer test". If it's positive, then you can get x-rays done (which may or may not show anything) and possibly ultrasound to see how bad the damage is. Then you'll know if you need to consider surgery.