"On the first two days," he said, "the Ferrari did not look good."
But Brazilian Massa ultimately went on to set the fastest laptime of the entire four-day test.
"Obviously, they found a solution to the problem," Rosberg surmised, adding that the 2013 Ferrari ultimately seemed "quite good".
But perhaps not as good as the Red Bull or McLaren, he added.
"They drove as though on rails," said Rosberg.
Ferrari's early struggles could be explained by a delay to the F138's development programme, caused by the push to the death for the 2012 title.
"If we talk about the aerodynamics of the F138," technical director Pat Fry is quoted by Russia's f1news.ru, "we started about three months later than we would have liked.
"We will do everything to make up for this delay," he added.
Fry referred to a stream of forthcoming "innovations" for the new red and black car.
One of them may be a mysterious hole beneath the nose tip that emerged in the final moments of the Jerez test.
"We will see at the next test in Barcelona if the hole is there or not," read a report in Italy's Autosprint.