"The campaign is in very good shape. We are organised. We are moving forward," Paul Manafort told Fox News.
US media outlets reported the campaign was in turmoil after Mr Trump repeatedly attacked the parents of a fallen US solider.
Staffers told CNN that they "feel like they are wasting their time".
However, Mr Manafort blamed those reports on Mr Trump's opponent Democrat Hillary Clinton.
"This is another Clinton narrative that she put out there and the media is picking up on," he said. "Mr Trump has appeared this week at crowds that are overflowing into the street."