Walker again dealt with pain in that ankle every time he dug deep to push hard off the rubber with his fastball and manager Scott Servais lifted the right-hander after 88 pitches and Seattle holding a 1-0 lead at the time.
"I just felt it a little after the first at-bat and it didn't really get any better," Walker said. "It happened in the first inning and just kept getting worse."
"He really competed his tail off," Servais said. "He gutted it out that fourth and fifth inning. I would have liked to have kept riding him because he was getting them out, but the information I got, everybody felt it was best to get him out of there at that point."
Walker was pulled after 3 1/3 innings in his previous start against Tampa Bay because of the same issue and Servais acknowledged it's a growing concern, which could be a tough issue for a team that already has starters Felix Hernandez and Wade Miley on the 15-day disabled list.
"Yeah, no doubt," Servais said. "I thought he gave us a great effort for what he had. But you could start to see the mechanics changing a little bit. And we don't want to hurt his shoulder or anything."
Servais said he didn't know if the 23-year-old Walker might be ready to make his next scheduled start in five days when Seattle opens a nine-game homestand against the Cardinals. Walker sounded more optimistic, but knows it depends how his rehab goes this week.
"We're going to do what we did last time and maybe a little extra and see how it goes," he said. "It felt really good leading up to today and even in the bullpen. But I think just going out there and the extra intensity of really getting after it and pushing off just kind of made it start barking again."
Walker had to dig deep to pitch out of trouble early and often against a tough Red Sox lineup. He stranded runners on second and third in the second inning by striking out Travis Shaw looking on a 96-mph fastball and getting Christian Vazquez to line out to right to end the frame.
Walker escaped damage again in the third after Mookie Betts and Dustin Pedroia opened with singles to put runners on the corners. Walker got Xander Bogaerts to fly out to center, with Betts holding at third rather than testing Leonys Martin's arm, then induced a 3-6-3 double-play grounder from David Ortiz to keep the game scoreless.
But those difficult innings came with a price on Walker's foot as the problem is heightened when he reaches back for extra juice on his fastball.
"If I don't push off, I use all arm," he said. "Being in situations where I had to really get after it, they had runners in scoring position and every inning was stressful."