Three of Ben Carson's top campaign managers stepped down from their posts last week, leaving the Republican presidential hopeful scrambling for more people to run his team.

His campaign manager Barry Bennett, communications director Doug Watts, and deputy campaign manager Lisa Coen all resigned from their positions on Thursday.

During an interview with ABC's "This Week," Carson revealed that his staff resigned due to the "very substantial changes" he had planned. 

"I was going to make some very substantial changes and Mr. Bennett decided that he could not live with those changes and that's okay," Carson explained. "It doesn't diminish anything that he's done. I think he's done a fantastic job."

"When things are not working the way you want them to, you analyse them and you make the appropriate changes in order to be able to accomplish your goals," he said.

The retired neurosurgeon said he was very pleased with the "spectacular" work the three former campaign brains were able to do. However, he said he is now in a "different ballgame and we need the ability to execute and not just have good ideas."

As for his business manager Armstrong Williams, Carson said the guy made some "bad judgments" and this is the exact reason why he wanted to implement drastic changes. "There's no question about it," Carson said. "But, you know, he's a friend. I think he's a valuable individual."

"We can't have people working at cross purposes, and that's one of the things that has to be fixed and is being fixed and is fixed, in fact, at this point," he added.

Both Bennett and Watts are pointing their fingers at Williams as "the factor" for the staff revamp. Even though Williams is not associated with the campaign, he is one of Carson's closest advisers and has been with him even before Carson announced his plan to run for president.