After eight years in red, Brazilian Massa's move to the once-great British team Williams was seen as a clearly backwards step for the 11-time grand prix winner.
But after pre-season testing, Mercedes-powered Williams has emerged as one of the strongest forces for 2014, with Massa tipped as an outside chance for outright victory in Melbourne.
"I've talked with Felipe several times this year already," Spaniard Alonso is quoted by Brazil's Globo.
"He is happy, Williams has so much history in Formula One and is not just any team, and they have had a very strong pre-season," he admitted.
"It is very positive because now they can be fighting. Felipe will definitely be a strong rival this year," added Alonso.
Alonso also played down the constant suggestions that both he and Ferrari will struggle to cope with his rivalry with Massa's 2014 replacement, fellow champion Kimi Raikkonen.
"We are trying to work together, working for Ferrari and arriving in Australia to fight for the race," Alonso said.
"So let's have this internal competition - normal in every team - because we all want to be ahead of the other one, and hopefully this can help Ferrari," he added.