Walker, who was placed on the disabled list last month due to inflammation in his pitching shoulder, was complaining that his shoulder felt stiff prior to Tuesday's game with Triple-A Tacoma.
"He just says he felt stiff," Mariners general manager Jack Zduriencik told reporters, according to MLB.com. "What you don't know about this is if you sleep on it wrong or however things work like that. When a guy's stiff and he's a guy like this, you just figure let's take as many precautions as we can."
Walker, 21, was expected to rejoin the Mariners' rotation next week. The 6-foot-4 right-hander went 1-0 with a 3.60 ERA in three starts with Seattle last season.
Zduriencik said Walker would be evaluated again Wednesday before the Mariners decide the next step in his rehab.
Walker was ranked earlier this offseason as baseball's No. 16 overall prospect by ESPN Insider Keith Law.
"You're talking about a long season and a long career," Zduriencik said. "So as much as we'd like to see him out here -- we're all excited about it -- but if he's out here the next day or two days from now, we'll continue to progress as scheduled."
The Mariners have dealt with multiple injuries to their rotation already this season. Aside from Walker, right-hander Hisashi Iwakuma (finger) also has been on the DL since March. The Mariners also placed promising left-hander James Paxton on the DL last week with a back injury.
Information from The Associated Press was used in this report.