In a statement released by the team, Tomlin said Roethlisberger will undergo the surgery at some point this week after having an MRI on Sunday night.
Later Monday, Roethlisberger said in a statement that the news was "shocking and heartbreaking" and that he feels "like I am letting down so many people." The quarterback said he plans to return to play next season.
"The Steelers committed three years to me this offseason and I fully intend to honor my contract and reward them with championship-level play," Roethlisberger said. "I will do all I can to support Mason [Rudolph] and the team this season to help win games. I love this game, my teammates, the Steelers organization and fans, and I feel in my heart I have a lot left to give."
The injury news wasn't all bad for the Steelers on Monday. Running back James Conner said on 93.7 The Fan that tests showed no tears or any significant damage to his knee and he is confident he will be able to play Sunday against the San Francisco 49ers. Conner, along with Roethlisberger, was hurt during Sunday's 28-26 loss to the Seattle Seahawks.
The Steelers, who placed Roethlisberger on injured reserve Monday, have not yet provided details about the injury, and there is no timetable for when the six-time Pro Bowler will be able to play again.
Roethlisberger was hurt late in the second quarter of Sunday's loss, grabbing his elbow after attempting a pass on the team's final drive of the half.
Roethlisberger had complained of arm soreness coming off the Week 1 loss at New England, multiple teammates said Monday. One Steeler told ESPN that he texted Roethlisberger on Sunday night and the quarterback responded that the injury outlook was "not good."
Monday's locker room scene was unsettled. This is the first time in Roethlisberger's 16-year career that he is set to miss more than four games in a row.
"It's super weird. Really surreal," guard David DeCastro said. "You've got to have the football mentality that the next guy's up and move on, but obviously with a guy like that it's a little different. Easier said than done."
Orlovsky: The Steelers 'are in a world of hurt'
Dan Orlovsky proclaims the Steelers are bad even when Ben Roethlisberger is playing and "the Steelers can kiss their season goodbye" should Roethlisberger need elbow surgery.
Added wide receiver Ryan Switzer: "It sucks because he's the unquestioned leader on our offense. ... We've got 14 games left. We owe it to ourselves and this organization and this fan base to pull it together."
In Roethlisberger's absence, second-year quarterback Rudolph nearly led the Steelers to a fourth-quarter comeback Sunday in his first career appearance, completing 12 of 19 attempts for 112 yards and two touchdowns with an interception.
Now the Steelers will have to rely on Rudolph, a third-round draft pick in 2018, to help them overcome an 0-2 start.
As reporters addressed Rudolph at his locker, slot corner Mike Hilton blurted out, "Tell them you're built for this s---, Mase." The Steelers were impressed with Rudolph's poise and preparation in his first game action, and Rudolph expects to build off that momentum Sunday in San Francisco.
"I worked for this," Rudolph said. "I wanted to be in this position. That was the plan."
Rudolph added that he'll try to learn as much as possible from Roethlisberger, who is expected to be around the team for guidance. Roethlisberger arrived at the team facility around 2 p.m. ET on Monday.
"I'm not going to be someone I'm not," Rudolph said. "I'm a big prep guy. I'm not a guy who goes out and wings it."
Rudolph is the only healthy quarterback on the Steelers' 53-man roster, so the team will likely look to outside help. The Steelers can't sign Landry Jones, who spent five years with the franchise, because he's contractually committed to the XFL. Devlin Hodges was elevated from the practice squad on Monday.
Roethlisberger, 37, is the Steelers' franchise leader in passing yards and touchdown passes and has led Pittsburgh to two Super Bowl titles. He led the NFL with 5,129 passing yards last season and signed a contract extension this past March that runs through 2021.
The news had an immediate impact on the Steelers' odds at Caesars Sportsbook, where they went from 40-1 to 100-1 to win the Super Bowl and 18-1 to 40-1 to win the AFC.
The Roethlisberger injury also affected the team's odds to win the AFC North at Caesars, which dropped Pittsburgh to +1600 for the division title -- well behind the Baltimore Ravens (-200) and Cleveland Browns (+170).