Simoncelli died after suffering "serious trauma" to the head, neck and chest in the accident on lap two of the race, which was subsequently cancelled.
After losing control of his bike in turn 11, the 24-year-old Italian slid across the track and was hit by Rossi and American rider Colin Edwards - both of whom were left with no time to react.
The Gresini Honda rider's helmet came off in the collision and he was taken to the medical centre for treatment, but he was declared dead.
Seven-time world champion Rossi did not speak about the accident in its immediate aftermath, but last night paid tribute to his compatriot in a short Twitter message.
"Sic [Simoncelli] for me was like a youngest brother," Rossi wrote. "So strong on track and so sweet in the normal life. I will miss him a lot."