The surgery was performed by Dr. Bill Meyers in Philadelphia, according to the team. Beckham is expected to make a full recovery.
League sources told NFL Media that Beckham played through the core muscle injury since August. Despite the injury, the wideout played all 16 regular-season games and recorded 74 receptions for 1,035 yards and four touchdowns.
"I'd say it wasn't handled right," Mayfield said. "He's not able to run as well as he should be able to, as well as he knows. And that's frustrating for him. You can sense that's some of his frustration, where that comes from. So it wasn't handled the right way in our training room. So, it is what it is. His 'not 100 percent' is still good enough for us."
The Browns hope Beckham can return to his former self under new coach Kevin Stefanski in 2020. Cleveland hired Stefanski as the team's next head coach earlier this month.
Since entering the league in 2014, Beckham has compiled 464 catches for 6,511 yards and 48 touchdowns.