Can Keflex Treatment make MRSA appear better ??

My pre-teen son has just gotten a "preliminary" culture result of a Staph Infection of his knee but they don't know what kind yet. At the initial time of the culture he was given a prescription of Keflex and Mupirocin ointment. Over the weekend the infection has all but totally cleared up and he has no sensitivity in the area at all anymore and has never felt ill or feverish. I understand MRSA is resistant to Keflex, so is this a good sign that the final culture result should be a Non-MRSA infection because Keflex appears to be working so well ?? We are going nuts waiting 5 days for this culture and just need some encouraging news right now!!