McCullum also gave his support for close friend Stephen Fleming, who has replaced him as Test coach.
Fleming, 53, will not be in charge for the Pakistan series as Marcus Trescothick will step in, meaning his first assignment will be the tour of South Africa in December.
“I had a great time in my four years but now it’s going in a different direction, I think Flem will be excellent,” McCullum said.
“He’s a little bit like me, but better – that’s how I would sum him up. He’ll do a great job, and the ECB have done incredibly well to get someone like him to commit to English cricket for the period that he has. It’s an outstanding achievement.”
Fleming inherits a Test side that has recently struggled after a breath-taking start to the McCullum and Stokes era.
McCullum won 10 out of 12 Tests in his first year in charge but since then, England have lost 18 and won 17 of 37.
The duo will have to negotiate an increasingly busy cricketing schedule, and manage the priorities of players who play all three formats, which includes Brook.
“We talk pretty much every day, we’re close mates,” McCullum said of Fleming.
“We’ve done a lot together and he’s one of those guys who I trust completely. We both just want what’s best for English cricket.
“We want to set this up so that English cricket can be successful across all three formats of the game.
“That is going to take some give and take at times, we both understand that, but we will work very closely together and give everyone the best opportunity to get the success that we all want to see.”