I went through this with my 11lb pekingese. We kept him crated for a while immediately after the surgery, per doctor's orders. Pekingese aren't jumpers, so that wasn't a huge issue, but for the rest of his life we were very careful about not letting him go up and down steps.
One thing that happened to our pup was that he started to favor his other leg so much that he ended up ripping that knee too. It wasn't a complete tear so we didn't do surgery on that one, but it did continue to degrade his mobility. From what I understand, this is fairly common after ACL repairs.
We ended up with a wheelchair for him, which we'd use on occasion to help him get around. It worked pretty well, but he'd get caught on obstacles due to the wide wheels.
I know I'm not full of good news, but his quality of life was still good for the years he lived after the surgery. We just had to be really careful with him, and did a lot of holding him up when he peed, etc. It was worth it.
Not gonna lie, this is making me kinda sad.