Sanders, who has racked up 19 wins over Clinton, most recently topped her in West Virginia on Tuesday night. Clinton is still on track to be the nominee, however, because of her current lead in pledged and unpledged delegates.
"He wins, wins, wins and then everyone sits around a table and says he can't win," Trump said on "Fox and Friends."
Earlier, Trump unveiled a new nickname for Sanders, referring to him as "Crazy Bernie" in a pair of tweets.
"Big wins in West Virginia and Nebraska. Get ready for November - Crooked Hillary, who is looking very bad against Crazy Bernie, will lose," he tweeted.
"I don't want to hit Crazy Bernie Sanders too hard yet because I love watching what he is doing to Crooked Hillary. His time will come," Trump added.
Trump repeated the new nickname on "Fox and Friends," saying, "I call him 'Crazy Bernie' because he's not very good, but probably beating him would be easier" than defeating Clinton.
"Who's going to run against the socialist and lose?" he said.
Asked about polls showing that Sanders does better against him than Clinton in the general election, Trump said that's because he hasn't paid attention to him yet.
"I've never hit him. Don't forget. I haven't started on him. I haven't said anything about him," Trump replied. "Maybe I'm going to have to start."
Sanders campaign manager Jeff Weaver responded to Trump while speaking to CNN's John Berman and Kate Bolduan on "At This Hour."
"Senator Sanders doesn't run his campaign based on advice from Donald Trump. Let me say that from the get-go," Weaver said. "But let me also say this: This myth that long primaries somehow hurt the eventual nominee, I think we all saw in 2008 how that just was not true."
Both Clinton and Sanders lead Trump in hypothetical general election matchups, according to a recent CNN/ORC Poll