Paxton said the fingernail started tearing off in the third inning and eventually limited his pitch selection, though he didn't allow any runs in his final two frames.
"The fingernail just started to lift up again, same as last year, but not as bad," he said. "But it was just painful throwing pitches. I did everything I could and tried to battle. It hurt the most on cutters and changeups. That's why after that we just went to fastballs and curveballs."
Manager Scott Servais said it was too early to know if the issue would affect Paxton's next start, which would come against the Rangers at Safeco Field on Monday.
"To his credit, he gutted it out and stayed in there to give us five innings," Servais said. "But he has a pretty serious nail issue. He should be OK to make his next start, but we'll see."
Paxton, who is 4-5 with a 3.83 ERA in 15 starts, said he'll get a fake nail put on the injured finger on Wednesday.
"It's frustrating," he said. "But we're going to try to find a solution here so it doesn't happen anymore. We're hoping the fake nail will help it for the rest of the season and kind of tack that down and make it so it doesn't lift up on me. We'll see how that handles it."
Paxton gave up two runs in the second, then a two-run homer to Adrian Beltre in the third, but saw his team rally from that 4-0 deficit to take a 6-4 lead before trading leads again in the final frames.
"That was one crazy game," he said. "I know for myself, I left some balls in the middle of the plate and they made us pay. They're a good hitting team. And we did the same thing to [Cole Hamels]. It was just one of those crazy games, back and forth, and it's unfortunate we didn't come out on top."
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.