MIAMI -- An MRI revealed a partial tear of the lateral collateral ligament in Braves outfielder Austin Riley's right knee, landing the rookie on the 10-day disabled list Thursday (retroactive to Monday). He was scheduled to be checked out by the team doctor at Marlins Park before getting a second opinion from Dr. James Andrews on Monday in Pensacola, Fla.

"From yesterday to today, it's a big step that I feel a lot better," said Riley, who missed time last season with a posterior cruciate ligament injury in the same knee. "Hopefully, that's a sign that it won't be too much, but you never know."

Riley intially felt some discomfort on Tuesday while performing a mobility exercise for his hips during a workout he does once every series. He immediately let the training staff take a look. Riley was deemed fine and took batting practice, where everything felt normal. It wasn't until Wednesday morning that there was swelling and discomfort "at a higher grade."

Braves general manager Alex Anthopoulos called Riley's progress on Wednesday optimistic, but he didn't want to put a timeline on a return.

"The fact that he's feeling better, the swelling's gone down, he's walking around -- those are all good signs, but the medical staff said we need more time. We need the inflammation to continue to keep going down," Anthopoulos said. "We should know more by Monday. We know there's a tear there. To what degree, that could be debated. We'll see how it goes."

With Riley sidelined, the Braves have just three true outfielders on the 25-man roster: Adam Duvall, Ender Inciarte and Ronald Acuna Jr.

"[Johan] Camargo can go out there if need be," Braves manager Brian Snitker said. "We've got [Matt] Joyce who can fill in. If we need to put Charlie [Culberson] back there, Camargo can play short. We're covered OK."

Before Thursday, the Braves were considering optioning Riley, who had struck out in 41.6 percent of his plate appearances and batted .169/.221/.324 since the beginning of July, because Dansby Swanson was expected to be activated from the injured list this week.

But the Braves' regular starting shortstop, who went on the injured list on July 24 with a bruised right foot, remains in Atlanta and will need to appear in rehab games because of the time missed.

"He wants to be out there," Anthopoulos said of Swanson. "He was progressing, moving in the right direction, but he's still not there yet. We still have two months left. When we get him back, we want to make sure he's back to stay. In talking to the trainers, they're optimistic that it should be maybe he's going to get another week or so of rest."

In a corresponding move for Riley, Atlanta recalled right-hander Jeremy Walker from Triple-A Gwinnett. The Braves will be playing with a short bench, but likely not for long.

Walker, whose first Major League stint came at the end of July, slid into a long-relief role on Thursday, allowing one run over 3 1/3 innings in the Braves' 9-2 loss. It gave Atlanta vital innings after Josh Tomlin threw 31 pitches in Wednesday's 11-7 win and both Luke Jackson and Anthony Swarzak appeared in each of the last two games.

"So it's just a little cover," Snitker said. "We'll see where we're at tonight. It may just be a one-day thing."