"Of course, I have a lot of respect for the people in our team but they were distracted, not focused enough," he told reporters.
Some believe Whitmarsh, who it is believed is not speaking to the media while his payout is negotiated, was regarded as 'too nice' to effectively run a F1 team.
"Martin is a friend," Dennis insisted. "We did not fall out. Some decisions you take in life are not that easy, and I will not elaborate."
What Dennis has found in Whitmarsh's wake, however, is the loss of Lewis Hamilton and title sponsor Vodafone, and Whitmarsh's new management pick, Eric Boullier.
Dennis insisted, however, that he separately also approached Frenchman Boullier, the former Lotus boss.
"For some time I had been discussing with the shareholders that things were not working," he said.
Another perhaps not entirely Dennis-esque choice was the rookie Dane Kevin Magnussen, son of the erratic former McLaren test driver Jan.
"I had reservations (about Kevin)," Dennis admitted.
"I'm not a great believer in sons of drivers," he added, but he thinks Kevin's "steely determination" will result in "an exceptional career".
So the new Dennis era has begun, and a first major input has been to turn down some "stopgap" solutions following title sponsor Vodafone's exit.
"I strongly believe we are (like) Manchester United," he said.
"Inevitably, when you have a run of poor results, people push the rate card down.
"But I know what this company is and what this grand prix team can achieve, and that requires the correct recognition from and close relationship with a principal sponsor," said Dennis.