Simoncelli, 24, lost his life after coming off his bike in a crash involving Colin Edwards and Rossi himself just minutes after the start of the race at the Sepang track.
Rossi, 32, and Simoncelli were known to be close and the Italian icon took to Twitter to express his sorrow at the death of his friend, who he referred to using Simoncelli's popular nickname, Sic.
"For me Sic was like a younger brother, as tough on the track as he was gentle in life," said Rossi. "I still can't believe it, I miss him so much."
Rossi was reported to have been in tears after the race when he heard the news.