X-rays were negative, and the Mariners announced that Paxton had a bruised left forearm.
"We'll know more in the next coming days and how the swelling is," Servais said. "It's unfortunate, but looking on the bright side, we probably got a little lucky. It could have been a lot, lot worse."
Felix Hernandez, who was demoted to the bullpen last week amid a season full of struggles, replaced Paxton in the first relief appearance of his career after 398 starts. He allowed two runs in 5⅔ innings.
Hernandez's stay in the bullpen might not last long. He's likely to take Paxton's turn in the rotation until the left-hander returns, Servais said.
"Paxton has been a horse for us all year. He's had a great season," Servais said. "But there's nothing you can do. It was a comebacker that was hit really hard. I knew right away when I went out there it wasn't going to be good."
Servais and a team trainer rushed from the dugout to check on Paxton.
"It got all muscle," Paxton said. "It's just swollen right now. It tightened up right away. By the time I walked off the mound it was starting to puff up."
Paxton is 10-5 with a 3.68 ERA this season. He entered the day 11th among qualified starters with 3.6 wins above replacement, per Fangraphs.
Hernandez walked Khris Davis before he pitched out of the jam in the first, getting a double-play grounder from Matt Olson. Hernandez ran into trouble in the third after a two-out walk to Chapman. Lowrie followed with his 19th homer this season.
"It was weird," Hernandez said of pitching in relief. "I just grabbed the ball and tried to throw as many pitches as I could, trying to get loose. I know my body, I know my arm. I was ready to go.''
Seattle is 3½ games behind Oakland for the second American League wild card.
Information from The Associated Press was used in this report.