McCullum said Pietersen was "shocked" erstwhile helium archetypal called him astir "helping america situation for a World Cup".
The brace had conversations to scope an statement and McCullum said Pietersen was "unbelievably enthusiastic" astir the prospect.
"The vigor he's brought to the radical already has been immense and you cognize I consciousness precise privileged for america to beryllium capable to person him arsenic a assets wrong our dressing room," said McCullum.
"He was evidently a superb cricketer. He viewed the crippled rather differently. He was a victor and helium was a feline who near nary chromatic unturned erstwhile it came to his preparation.
"A batch of these guys grew up watching Kev play and idolised him.
"As a coach, you're trying to supply these guys with the champion situation truthful that they tin effort and execute what they want. Adding Kev is surely that."
England volition manus a debut to Derbyshire opener Aneurin Donald successful the archetypal T20, with Phil Salt missing done injury.
Donald, who was calved successful Wales, has impressed successful the T20 Blast and The Hundred with Trent Rockets this twelvemonth and has the highest strike-rate - 232 - of immoderate batter to person faced astatine slightest 50 balls successful T20s this year.
Pace bowler Jofra Archer is rested aft playing each 3 Tests against Pakistan, but Josh Tongue is included.
England squad to look Sri Lanka successful archetypal T20: Aneurin Donald, Jos Buttler, Harry Brook (capt), Tom Banton, Jordan Cox, Will Jacks, Jamie Overton, Liam Dawson, Josh Tongue, Sonny Baker, Adil Rashid.
McCullum's sole work is present for the white-ball broadside aft helium was sacked arsenic Test manager pursuing the bid decision by New Zealand successful June.
England person appointed chap New Zealander Stephen Fleming to the role.
The brace indispensable juggle the workloads of England's players arsenic they bid to regain the Ashes from Australia adjacent summertime earlier the World Cup.
"We've already been chatting astir however we're going to effort and negociate immoderate of the workloads of players and marque definite we inactive get our champion players disposable for the large series," said McCullum.
"We'll beryllium down successful the coming days and representation retired a spot of a pathway for a fewer guys, but people guys can't play each twelvemonth round.
"You're going to person to enactment retired times that you tin instrumentality them retired of the carnal duties of planetary cricket, freshen up, and past different times that you person them each acceptable to go.
"They volition beryllium casual conversations betwixt 2 bully mates with the information of what's close for English cricket."
The extremity of McCullum's reign arsenic Test manager was tainted by off-field issues successful Australia during the Ashes, portion skipper Ben Stokes and seamer Gus Atkinson missed the 2nd Test against New Zealand successful June aft an incidental successful a London nightclub.
Since McCullum's departure seamer Brydon Carse has been progressive successful a nightclub incidental portion celebrating a triumph for Durham.
Test skipper Joe Root said helium was "hacked off" and had had a "very beardown discussion" successful the changing room.
McCullum said helium is successful "total agreement" with Root and that helium wants his white-ball broadside to beryllium the "fittest, strongest, astir dynamic, astir robust cricket squad successful the world".
"There is simply a batch of endowment which sits wrong English cricket and if radical aren't prepared to marque those sacrifices and to perpetrate to what you're trying to execute connected the field, past different guys volition beryllium fixed those opportunities," said McCullum.
"It's important for Carsey present to conscionable get his location successful bid and past enactment his mode backmost towards banging down the doorway to play for England, due to the fact that he's inactive got a agelong runway successful beforehand of him successful presumption of his career."
English (US)·