The Spaniard was revealed as McLaren's latest signing in November, after speculation raged he would depart the Italian garage - with which he was runner-up in the drivers' standings three times in five seasons.
Brazilian Massa, team-mate of Alonso at Ferrari for four seasons, said the relationship between the famous racing team and the two-time championship winner had deteriorated.
"Everybody knew he was going to leave Ferrari, so it wasn't exactly breaking news," Massa told Autosport.
"Maybe we saw that Ferrari didn't give him the best car that he expected, but maybe also he didn't help Ferrari to grow.
"So maybe now is the time for him to change."
Massa, preparing to embark on his second season with Williams, said he cared more for Ferrari's fortunes than Alonso's, after his career at the garage was spend largely in the shadows of the number one driver.
"If it [Alonso's defection] is going to be any worse or better for Ferrari, I don't know," Massa said.
"Fernando is perhaps the best driver on the grid, so we don't know if it will be enough for Ferrari to make the step forward.
"We need to wait and see. But I just hope for the best for Ferrari, anyway."
Massa will link up with Valtteri Bottas again in the 2015 season, after the Finn outscored him by 52 points in the 2014 campaign.
Without a win in six seasons despite 12 podium finishes, Massa said his partnership with Bottas was a win-win.
"He [Bottas] was good for me, but what was good for him as well is that our driving style is not so different," Massa said.
"So many races we had a very similar set-up and if something was working for me, then he could see, and it was the same the other way around."