Zeus had a similar problem earlier on in the year after straining himself chasing a frisby on the beach. The limp was noticable but not severe, we took him to a vets in Porthmadog as we were on holiday, he checked his shoulder and diagnosed a strain, he was prescribed a 7 day course of Rimadyl, he advised us to reduce the amount of tablets if Zeus improved, we were told only to allow gentle on lead exercise and he told us to visit our own vet if he was no better a week on. It didn't take long to heal, he didn't finish all of the tablets and no follow up was required.
He was most put out at not being allowed to play with his frisby for the rest of the holiday though, every morning he woke me up early and would sit by the awning door waiting with his favourite toy looking up at me with his big brown eyes....I felt sooooo sorry for him bless