The price target for McDonald's also was lowered to $195 from $224 at Piper Jaffray.
The stock fell 2.93% to $188.25 in trading Monday. The stock gained 96% with Easterbrook at the helm since March 2015.
"Our experience leads us to take a more cautionary view noting the potential lack of momentum and time involved in formalizing a new team," Regan added.
Analyst Andrew Charles at Cowen said Easterbrook has been well-regarded and he expects his departure to weigh on shares. But he added, "We are not expecting any major changes in strategy."
Cowen rates McDonald's at outperform with a price target of $230.
Easterbrook was fired after the board's finding that "he violated company policy and demonstrated poor judgment involving a recent consensual relationship with an employee."
Easterbrook also resigned from the board of the burger giant.
He sent an email to employees on Sunday saying that he violated company policy.
"This was a mistake," Easterbrook wrote. "Given the values of the company, I agree with the board that it is time for me to move on."
Chris Kempczinski, president of McDonald's USA, will take over as CEO, McDonald's said. Kempczinski has also been elected to the McDonald's board.
Kempczinski told The Wall Street Journal in an interview that he would maintain Easterbrook's focus on technology as CEO and believes the company's investments will pay off.
"There isn't going to be some radical, strategic shift. The plan is working," Kempczinski said.
Cowen's Charles said Kempczinski was the most logical person to replace Easterbrook, given the importance of the U.S. market to the company.