I read about it somewhere and I'm trying to remember the details, but it seems to me that Urso didn't want to play "Early Autumn", or wasn't playing it the way Herman wanted.
I know how it sounds, but be fair to Carl, I think he really was coming in to take Green's place for a week or two, as Urbie was away for a family emergency or something. And of course, when Urbie came back, Woody simply kept Carl on (and who wouldn't?)