Pushing for their first postseason appearance since 2020, the Athletics lost their leader and unofficial captain as manager Mark Kotsay announced on Wednesday that two-time All-Star Brent Rooker will undergo season-ending knee surgery.

Kotsay said that the 31-year-old designated hitter will have a procedure to address a cartilage tear in his left knee at a later date.

"This is a big blow," Kotsay said. "It's a middle-of-the-order bat. It's a guy that produced for us offensively for the last three seasons with 30-plus homers. In terms of replacing Rook, there's not one guy that is going to come in here and step in with that type of production. We'll do our best to fill that void and make the best of the situation."

Injuries this season have taken a toll on Rooker, who entered 2026 with 99 home runs over the previous three seasons, including a career-high 39 in 2024, when he also drove in a career-high 112 runs.

In April, he sustained a strained right oblique that kept him out for 15 games. Upon returning April 25, Rooker hit just .216 with a .709 OPS and eight homers in 36 games before the knee issue surfaced, sidelining him since the June 8 game. A cortisone shot on June 10 did not do the trick.

"There's not one instance of the injury," Kotsay said. "This could have been something that just over time continued to deteriorate. The cartilage in itself caused that bone bruise. That's from a lack of cartilage in the knee. We tried the conservative measure of doing the injection. Obviously, with the baseball activity increasing, it just continued to be an issue."

Rooker ends his season hitting just .200 with a .670 OPS, 10 home runs and 29 RBIs in 48 games.

The A's will also miss Rooker's presence and leadership, especially for the younger players who often turn to him for hitting tips. The slugger will not join the team on the road, but he is expected to be present and available to offer his expertise at home games.

"In terms of the leadership, that's not going to change," Kotsay said. "He's still going to be a part of the group on a daily basis. When he has the surgery, he'll be rehabbing here with the team so he can still provide that leadership."

Joey Meneses, who was thriving at Triple-A Las Vegas before a recent call-up, is among the players likely to fill the DH spot.

Rooker began his major league career in 2020 with the Minnesota Twins, who selected him with the 35th overall pick in the 2017 draft. In 573 games with the Twins, San Diego Padres, Kansas City Royals and Athletics, Rooker is a .254 hitter with an .815 OPS, 119 home runs and 322 RBIs.