The seven-times world champion scored his best result on the Italian bike at Misano last time out, when he finished second.
But he added that more time was needed to see the true extent of progress Ducati has made since introducing a new chassis and swingarm.
"This is a big test for us and will test better our data because in Misano it was a good weekend and a great race result for us," he said about the San Marino MotoGP weekend, much of which took place in wet conditions.
"Now in the next four races we the chance to see if we have made a step forward in reality.
Rossi admitted that private testing at Misano immediately after the weekend had revealed further information.
"We found something that I didn't like, but also something else that looks better compared to Sunday," he said.
"So we will test tomorrow and if the conditions are good we will try to understand if it is a small step."