NBC
In an interview with NBC's "Meet The Press" that aired on Sunday, the real-estate magnate said that he was surprised by Ryan's announcement, given that the two had spoken on the phone several weeks ago.
"I was blindsided a little bit, because he spoke to me three weeks ago, and it was a very nice call, a very encouraging call," Trump said of Ryan.. "I was doing well. He called me, I think, to congratulate me about [the New York primary]."
He added: "Then all of a sudden, he gets on and he does this number. So I'm not exactly sure what he has in mind."
The House speaker announced on Thursday that he "was not ready" to endorse Trump, saying that the presumptive nominee needed to "set aside bullying, set aside belittlement" and develop a campaign platform that could unify the Republican party.
"I think what is necessary to make this work, for this to unify, is to actually take up principles and advance them. And that's what we want to see," Ryan said on CNN.
Over the weekend, the Trump campaign repeatedly criticized Ryan for his non-endorsement.
In an interview with Business Insider on Friday, Trump campaign spokeswoman Katrina Pierson questioned whether Ryan should be part of the Republican leadership if he could not support the presumptive presidential nominee.
"We've heard people like Paul Ryan and others say, 'Donald Trump doesn't represent my party.' Well then, you shouldn't be in the leadership, because all of the voters have selected Donald Trump," Pierson said.
The campaign spokeswoman criticized Ryan's involvement in passing last year's complex bipartisan spending package, which increased federal-government spending, but also included some sought-after tax breaks.
"This is the Paul Ryan Republican Party that [Trump's] talking about. Paul Ryan just patted himself on the back for passing a $1 trillion spending package. And they want to lecture the voters about being fiscally responsible and conservative?" Pierson said.
Watch the interview below, via NBC: