GLENEAGLES, Scotland – Stacy Lewis hurt her lower back at the Cambia Portland Classic in her last start nearly two weeks ago.

“It started bothering me there,” Lewis said Tuesday after withdrawing from the U.S. Solheim Cup team. “I had some tests and treatment last week, hoping it would go away, and it felt a little better for a while, but ...”

But Lewis knew her growing pain this week might jeopardize American chances, so she told U.S. captain Juli Inkster on Tuesday morning that she was out.

“I didn't want to have to make that decision, but it's what's best for the team,” Lewis said. “And what's best for me going forward, with my body.”

Ally McDonald, the first alternate, will step in and make her first Solheim Cup appearance. That gives Inkster a whopping six Solheim Cup rookies on her roster. That’s half her team. That’s the most rookies the Americans will field since the inaugural Solheim Cup in 1990.

McDonald just missed being one of the eight qualifiers who made the team on points, finishing ninth.

“Stacy's been very up front, very honest with me,” Inkster said. “She realizes it's a team event. She realizes we need 12 players. And she realizes also that Ally needs to get a feel for the golf course.”

Lewis called Inkster last Thursday to alert her there was an issue. Inkster immediately called McDonald, who was planning to wait until the middle of this week to make the trip to Scotland.

“I told Ally it was critical to get her butt here on Sunday with the rest of the team,” Inkster said.

Solheim Cup rules don’t permit an alternate to play the course. Lewis knew trying to play through the injury wouldn’t help the American cause.

“It’s just not fair to the teammates,” Lewis said. “I don’t want to put Juli and the girls in a situation where they’ve got 11 [players] on Sunday, because Sunday’s the most important day of the week.”

That happened in Ireland in 2011. Cristie Kerr started the week with wrist pain and tried to play through it. She ended up withdrawing Sunday morning, before singles play. The Americans conceded her match and a point and lost the cup, 15-13.

Lewis believes her injury may be musculature, but she isn’t sure. She has undergone a series of X-rays, MRIs and ultrasounds over the last week and doctors still aren’t certain what is causing her pain.

Lewis was born with scoliosis and underwent a complex surgery to remedy her issues coming out of high school, with doctors fixing a rod and five screws to her spine. Remarkably, she hasn’t missed a golf tournament because of back pain since college.

“When I'm walking around, it doesn't really hurt, but the rotation of the golf swing really just fires it up,” Lewis said of this new injury. “I think it's one of those things where it needs rest, more than anything.”

Inkster set Tuesday as the “drop-dead deadline” for Lewis to decide whether she could play.

“Because if Ally was going to have to play, she needed time to prepare,” Inkster said.

Lewis, who walked side by side with McDonald through most of Tuesday’s practice round, will stay the week, as a sort of unofficial assistant captain.

“I came in mentally prepared to step in as a player or even as a cheerleader,” McDonald said.