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Usually if the tear is bad enough, they don't want to bear weight with it. I would let your vet know what is going on and discuss it, but since he has made that much improvement so quickly, I bet it will be okay if you let some time pas and let him rest to see how much improvement you get. I doubt it is a complete tear at any rate or he wouldn't be able to use it. Try to keep him off of it so it can heal and if at any time it gets worse or he seems painful just take him in then.
Please let me know if you have more questions,
The tibia isn't actually in a socket like the hip is. It is held together by the cruciate ligaments and the small knee cap and patellar ligament run over the top of the joint. You don't really see a deformed look to it but it becomes "loose" or moves in ways it shouldn't be able to move if the ligaments are torn. It is very painful if this is the case and most dogs won't put weight on it at all. If he is improving in this short amount of time, most likely there isn't a serious tear.
If it is just a sprain, most of the time it heals with time and rest. If it is a more serious injury such as a tear of the ligament, it can cause some arthritic changes in the knee.