"I knew there wasn't a broken bone, but we're worried about whatever ligaments and tendons are in there," Kiermaier said. "I'll get an MRI tomorrow morning and get further evaluated and let the doctors tell me what they think is going on."
Rays manager Kevin Cash wasn't optimistic about Kiermaier's status.
"Not too excited about the potential of it," Cash said. "I don't want to get too far ahead of it. But he's going to be out."
Kiermaier singled to left with one out in the first inning. When Phillies left fielder Rhys Hoskins bobbled the ball, Gold Glove center fielder took off for second base.
"I saw him bobble it, so I took the base," Kiermaier said. "The opportunity was presented to me. I go in there and knew it was going to be a close play."
Kiermaier slid head first into the bag and came away shaking his right hand afterward.
"The hand just kind of got caught in the ground," Kiermaier said. "Thumb went in a weird angle. Just one of those things where I felt like I had the chance to overslide the bag, like I've done a couple of times in my career. And I was just trying to hold on.
"I don't know if that's what made my thumb go in the angle that it did. I thought I jammed it right from that moment, but I didn't like the way my thumb felt after that."
After the Rays' training staff tended to Kiermaier, he remained in the game, scoring the Rays' second run on Mallex Smith's triple. When the Rays took the field in the second, rookie Johnny Field took Kiermaier's spot in center field.
"I didn't like the way it looked, and I didn't like the way it felt," Kiermaier said. "Initially, I thought I just jammed it."
After he squeezed head athletic trainer Joe Binge's hand, Kiermaier didn't like how his thumb felt. After scoring, he went into the tunnel behind the dugout to see if he could throw a baseball.
"And I didn't even need to throw," Kiermaier said. "By just holding a ball, it's hard to explain what my thumb was doing. I just know something isn't the way it's supposed to be in there."
Cash called the situation "beyond frustrating."
"We're all tired of K.K. getting hurt," Cash said. "And there's nothing he can do about it. He goes about his business as hard as anybody in the game. It was a typical K.K. play, busting it around to second base, the guy bobbles the ball.
"You'd like to see him get a little more reward for that than missing significant time. But this is three years running where there's a chance he's going to miss a chunk."
Last season, Kiermaier missed 61 games while on the disabled list with a right-hip fracture. In 2016, he missed 48 games while on the disabled list with a fractured left hand.
Kiermaier expressed regret and frustration about letting down the fans, teammates and the organization by getting injured.
"Now it's just about getting better and getting back on the field as soon as possible," Kiermaier said.
Bill Chastain has covered the Rays for MLB.com since 2005.