“While braking for the first corner, I entered a bit long, and although I tried to stay to the inside, I lost the front and couldn't stay up. I'm sorry, because I also took out Stoner, and I certainly didn't want to do that.
“I apologised to him, and I'm truly sorry; it was a mistake. It's a shame because we really could have gotten some satisfaction, but we'll keep trying. We're still not so fast in the dry, but we're working hard.
“Anyway today's fifth place gave us eleven points that are very important in the championship.”
The last time Rossi crashed, and brought another rider down with him, was when he collided with Marco Melandri at the 2005 Japanese Grand Prix.
“It's a shame about the crash because he would have fought for the win, and obviously, we're also very sorry that Casey was involved,” added team manager Vittoriano Guareschi.
Lorenzo went on to take victory on Sunday, after Simoncelli crashed out of the lead, and now leads the world championship by nine points over Dani Pedrosa.
Stoner, Ducati's 2007 world champion, is 20 points behind in third with Rossi a further five points adrift in fifth.
It wasn't all bad news for Ducati on Sunday - Rossi's Nicky Hayden took third place and with it the factory's first podium of the season.