With my first tripawd, Yoda, it was easy.  He wasn't the type to do much contact play with other dogs anyway – he just liked to run with other dogs, otherwise he'd only do brief greetings then do his own thing.  That made it a much easier decision to start bringing him back to the park, and it was barely a month when he first did.  I never had any park problems with him in the four months of good health he had before osteosarcoma overwhelmed him and we probably went an average of twice a week.  But Gerry is a different story, though I did bring him to the park just a month after amputation also, and he did great initially.  He had fun and was polite to other dogs – didn't slip or fall even though there was snow and some ice.  I hadn't had Gerry for long before his amputation, and he already had the injury that led to his needing an amputation, so I don't know if his behavior was any different pre-injury at the dog park, but I suspect not.  Over time I've learned that Gerry does seek out contact-play with other dogs and actually seems attracted most to dogs who will get aggressive with him and Gerry too isn't the type to back down once that starts either and these encounters were occurring every one out of three trips to the park.  Fortunately, I was always able to break them up before any injuries resulted, but it was only a matter of time.  So, sigh, we had to stop going all together.  We are going to try a new class soon being offered in my area called "Dog Park Skills."  I don't know if it'll work for Gerry's issue, but it couldn't hurt. 

What has been recommended to me previously is to work super hard on "come" so that your dog loves to come running every time you call without hesitation.  That way if you see a possible problem situation at the park, you can get your dog away from it immediately.  Gerry and I got a lot better at it, but it'll take a lot more to get him to ever come when he sees one of those tough dogs he likes.  For what it's worth  Good luck!