"I think the question isn't, 'What's going to happen now?' The question is, 'What's going to be the day after the last primary?' " Messina said during an appearance on MSNBC's "Morning Joe."
"People forget that Hillary Clinton flew to New Hampshire, endorsed Obama and started campaigning for him very quickly. And so the question is how quickly Bernie will do that," Messina added.
"I think primaries are always difficult and everyone always freaks out, but this is going to be OK," the former Obama aide said.
Clinton took a similar tone during an interview Sunday on NBC's "Meet the Press," stressing the need "to try to unify the party." "I did that when I lost a much closer race to Sen. Obama," Clinton said, referring to the 2008 Democratic presidential race.
Sanders has so far ignored calls from Democratic leaders to bow out of the race in an effort to allow Democrats to focus on presumptive Republican presidential nominee Donald TrumpDonald John TrumpMnuchin knocks Greta Thunberg's activism: Study economics and then 'come back' to us The Hill's Morning Report - House prosecutes Trump as 'lawless,' 'corrupt' What to watch for on Day 3 of Senate impeachment trial MORE, who has closed the gap with Clinton in recent national polling.
Tensions among Democrats have escalated recently, with Sanders vowing to fight on until the national convention in July, while feuding with Debbie Wasserman Schultz, the head of the Democratic National Committee (DNC).
Messina predicted Monday that Sanders would back Clinton and supporters of the independent Vermont senator would come around to supporting her in an effort to defeat the presumptive Republican nominee.
"I think it's the right thing for the party. I think his supporters are going to look at Donald Trump and say there's no way we're going to allow this guy to be president of the United States," Messina said.

Messina touted Sanders's ability to "register new voters all across the country," but dismissed the notion that the senator's supporters may back Trump, another outsider candidate.

"Oh, come on. There’s not a chance someone who is going to vote for Bernie Sanders in a primary is then going to go to Donald Trump in a general election. I promise you, that’s not gonna happen," he said.