Morneau, who has also been sick with the flu in recent days, has been out with concussion symptoms stemming from a dive for a ground ball on Aug. 28.
The first baseman had hoped to return as a designated hitter before the season ended. But his latest battle with the flu set him back, and the club hasn't had a chance to evaluate his concussion symptoms the last few days.
"He's been sick and throwing up and had a headache, so we haven't had a chance to truly evaluate how he really feels," McWane said. "But the knee surgery is a very simple surgery."
Morneau said the knee had been drained last December, and was something he knew would need to be cleaned up after the season. The two surgeries should sideline Morneau no longer than a couple of weeks, he said.
Twins manager Ron Gardenhire said he wasn't surprised by the decision, as there are just 10 days left in the season, and the training staff had been leaning toward shutting down Morneau.
"They've been talking about that over and over again," Gardenhire said. "Then, he got the flu bug and that kept him away for a couple days, too. So we really haven't been able to sit down with him and talk to him -- other than over the phone. He came in yesterday, but hasn't been doing too well."
Morneau finishes the season with a .227 batting average, .285 on-base percentage and .333 slugging percentage -- along with four homers and 30 RBIs in just 69 games. He missed 56 games from mid-June until mid-August with a sprained left wrist, and also underwent neck surgery on June 29.
Morneau, who played in just 81 games last season after suffering a concussion before the All-Star break, has missed 201 games over the last three years. It's still unclear whether he'll be able to remain at first base next season, or if he'll be limited to serving as the club's designated hitter.
"It's not a lot of fun when you're physically not able to [play], when you have that desire. Whether you win or lose, it doesn't matter. You want to be able to go out there and compete," Morneau said. "Nobody plays this game at 100 percent. Everyone deals with injuries.
"There's certain stuff you can play with and certain stuff you can't. The brain isn't something you mess around with."