"Whether or not we have to DL him, I'm not sure," manager Bob Melvin said, "but there was an X-ray and he does have a fracture."
Stephen Vogt is the only other catcher on the 40-man roster, so the A's would likely search the waiver wire for some help, with no clear backup option in the Minor Leagues.
Norris went to block the plate when Seattle's Kendrys Morales hit a grounder to second baseman Alberto Callaspo, who threw home to try to prevent Nick Franklin from scoring, but the rookie slid around Norris' attempt to block with his left foot.
Tthe Mariners' rookie second baseman didn't escape unscathed, however, sustaining a laceration on his left knee that required five stitches. He's expected to miss four or five days.
Norris, who missed time last week with back spasms, was responsible for driving in the A's first run of the game, and he's batting .393 over his last 13 games after hitting .179 over his previous 53 contests.
"He plays a pretty integral role on our team," said reliever Ryan Cook. "What he does behind the plate is special, the way he works with us on the mound. The way he's been swinging as of late, obviously it'd be a big loss to any team."
"It's been tough for our catchers here recently," added Josh Donaldson. "He was struggling with his back, now his toe. It's one of those things where I don't know how it's going to play out."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.