"Donald is going to be as serious as Donald wants to be. And he's going to determine through the depth of his answers and the seriousness of his answers whether he is a serious candidate or he isn't," Christie said in an interview with CNN's Jake Tapper aired Sunday on "State of the Union."
"Anybody can do well for a month in this business, especially if you have talent and personality, and Donald has both those things. Let's see how this goes over the course of time," he said.
The straight-talking New Jersey's governor's presidential hopes largely ride on his confrontational style catching on -- particularly in New Hampshire, the state he's made the focus of his campaign.
But Trump's rise -- a new NBC News/Wall Street Journal poll out Sunday had the real estate mogul leading the Republican presidential field again with 19% support -- has dipped directly into Christie's potential well of support.
Now, Christie is landing around ninth place in national polls, putting him on the edge of missing the 10-person cutoff for Thursday's first GOP debate in Cleveland, hosted by Fox News.
That's fine with him, as long as he makes the cut, Christie said.
"Once you get on the stage it's not going matter whether you're number one or five or 10," he said. "You get the opportunity to make your pitch to the Republican primary voters across America."
He took another shot at Trump during the interview, calling his comment that undocumented immigrants should all be deported, with "the good ones" allowed back in an expedited process, as impractical as Trump's other proposal to build an enormous wall across the U.S.-Mexico border.
"This is like building a 2,000-mile wall across the border that Mexico's going to pay for," he said. "It sounds really good you, pound your chest, but the question is how? How are you going to do it?"