"In all likelihood, it will be Hillary Clinton," Sanders told CNN's Chris Cuomo on "New Day."
Sanders said he was focused on his leverage over the Democratic party platform.
"My job right now as a candidate is to fight to make sure that the Democratic Party not only has the most progressive platform in the history of the Democratic Party, but that that platform is actually implemented by elected officials," Sanders said on CNN.
He also declined to say whether the time will come that he fully endorses Clinton, saying he is waiting to see what she says about his priorities. He also would not say explicitly, when pressed by Cuomo, that she won the nomination fairly.
"I think the system has many, many flaws, but we knew what we were getting into," Sanders told Cuomo. "I'm not saying that they changed the rules. No, they didn't."
But he also expressed time and again that he was working to defeat Trump. Asked if the lack of a 100% answer meant he would vote Trump, Sanders said, "Oh, God no."
Earlier, in another TV appearance, Sanders was firmer in his support for Clinton.
Asked on MSNBC's "Morning Joe" if he would vote for the former secretary of state, Sanders said:
"Yes. Yes, I think the issue right here is I am going to do everything I can to defeat Donald Trump. I think Trump in so many ways will be a disaster for this country if he were to be elected president."
Sanders has not yet formally conceded the race for the Democratic presidential nomination, but Clinton became the presumptive Democratic nominee as superdelegates committed to the former secretary of state and she won primaries in New Jersey and California.
Since then, Sanders has been plotting his political future and held a series of meetings with top Democrats, including President Barack Obama and Clinton herself as the party works to unify after the contentious primary.
On CNN, Sanders said while he knows he hasn't run a campaign that won the Democratic nomination, he still sees the chance for victory of sorts.
"I don't have the votes to become the Democratic nominee, you know that, I know that, we're good at arithmetic," Sanders told Cuomo.
But he said if he can get the party to embrace free public college tuition, a $15 minimum wage and an aggressive battle with climate change, "you know what? I think I've run a winning campaign."