Ethier, along with Dodgers brass, decided to halt the All-Star's on-field activity on Thursday. The club will send the right fielder to Dr. James Andrews in Birmingham, Ala., for a second opinion on Friday.
"It sounds like this is it. I don't think he's going to play again [this season]," Mattingly said before the Dodgers doubleheader against the Nationals on Thursday.
Ethier played in 135 games this season, including the Dodgers' last game on Tuesday. He's batting .292 with 11 home runs and 62 RBIs.
Mattingly said the decision to sit him now is "just the fact that [the knee is] bothering him."
"It's kind of a group decision," Mattingly said. "Not that we thought it was going to improve, but at some point you get to the point ... get a second opinion, trying to get this process started so basically the rehab turns into a longer period of time where he has more time to make sure he's healthy, so he can do the work he wants to do during the winter."
Ethier, who hit 20 or more home runs in his previous three seasons, was ambivalent when asked to reflect on his season, instead focusing on the work that lies ahead.
"I'm just trying to figure this whole thing out right now, and figure out a way to get it better where it won't affect me for Spring Training," Ethier said.
Ben Standig is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.