The Steelers on Saturday acquired Boykin, considered a top-shelf slot corner, from the Eagles for a conditional 2016 draft pick.
"He can't relate and that makes him uncomfortable," Boykin told CSNPhilly.com of Kelly in a text message Sunday morning. "He likes total control of everything, and he don't like to be uncomfortable. Players excel when you let them naturally be who they are, and in my experience that hasn't been important to him."
Boykin said his differences with Kelly were based on lack of communication. Basically, he felt ignored.
"When you're a player, you want to be able to relate to your coach off the field," Boykin told reporters at St. Vincent College, site of Steelers training camp. "There were times he just didn't talk to people. You would walk down the hallway, he wouldn't say anything to you. I'm not saying he's a racist in any way."
Boykin said Kelly's inability -- or unwillingness -- to relate to players is probably why the discord exists for some.
"I felt a lot of guys in that locker room feel the same way," Boykin said. "Of course, when you're in the organization, you're not going to voice your opinion. For me, I've always been a guy of honesty. Not trying to put anybody out in any way, but if you're honest with me, I'll be honest with you, and I felt like that honesty wasn't there all the time."
Boykin said the public doesn't have to believe his stance but added there is credibility to former players speaking out about Kelly because "we're actually in the locker room." He then made it clear that he is not talking about Kelly anymore.
Kelly on Sunday told reporters that Boykin appeared "stunned" by the trade when the coach delivered the news.
"He was disappointed," Kelly said. "I think he really liked it here, very close with his teammates."
The third-year coach admitted the timing for Boykin wasn't ideal but that the Eagles had developed better depth at corner with three rookies and the Steelers had "actively pursued" Boykin, dating back to the draft. Kelly said the Eagles turned down the Steelers twice.
After sitting down with defensive coordinator Bill Davis, secondary coach Cory Undlin and personnel man Ed Marynowitz to consider the offer, Kelly decided it was better to get value for Boykin now rather than wait until the end of training camp.
"It speaks more to our depth at the position," Kelly said. "We thought it was the move to make."
Kelly said the offers from the Steelers kept getting better. The Eagles received a fifth-round pick in the 2016 draft. If Boykin plays more than 60 percent of Pittsburgh's defensive plays this season, the pick becomes a fourth-rounder.
"When he left here last night, he shook my hand gave me a hug," Kelly said. "I like Brandon."
Boykin said he has heard great things about the Steelers' culture. He is not sure whether the Steelers will ask him to play in the slot or on the outside, but the franchise is getting a "versatile player," he said.
ESPN Eagles reporter Phil Sheridan contributed to this report.