Simoncelli, jeered throughout the weekend after his controversial clash with Dani Pedrosa at the previous Le Mans round, began the race with one eye on victory - and a debut MotoGP rostrum at the very least.
But he simply wasn't able to replicate his practice pace and dropped 4.4sec away from Rossi on his way to sixth.
MotoGP rookie Cal Crutchlow, who had qualified sixth on the grid, fought up the order from a bad start to take seventh from Nicky Hayden on lap 10. A position he held to the line for his best grand prix result to date.
Loris Capirossi won a four-rider fight for ninth, on a day when Pramac team-mate Randy de Puniet was taken down after contact from Hiroshi Aoyama, who also crashed in the incident.
The MotoGP field was reduced to just 15 riders this weekend after Colin Edwards joined Pedrosa on the sidelines. Pedrosa broke a collarbone when he tangled with Simoncelli, while Edwards suffered the same injury during Friday practice in Catalunya.
Edwards wanted to at least start the race, then pull into the pits - thus keeping his perfect record of starts since his 2003 debut intact - but he was unsurprisingly declared unfit to ride.
The British Grand Prix at Silverstone takes place next weekend, when Edwards will doubtless try to return, while Aoyama is tipped to take Pedrosa's Repsol Honda seat.
Catalan Grand Prix: