The incident occurred in the first inning, marking the second time in as many days Maxwell has been shaken by a foul ball. After grounding out in his first at-bat, he was replaced behind the plate by Stephen Vogt to open the fourth.
He went through a mini series of concussion tests without any issues.
"I've taken a lot of hits in the last week or so, including a couple before I got up here, so I think it just kind of wore on me a little bit," Maxwell said. "I should be fine here in a couple days."
Said A's manager Bob Melvin: "It's a concern. We're going to keep an eye on him for another day or two. Nowadays, with any catcher, when you're taking one or two -- and I think he's had four or five in week, he had a couple in Triple-A, too -- you get nervous.
"When I was giving him signs in between innings before he came out he was looking at me longer than he normally does, which means it took a little while to process for him. Once that was going down we felt precautionarily we needed to get him out."
The A's already have one catcher on the concussion disabled list: Josh Phegley, who is expected back Friday.
Jane Lee has covered the A's for MLB.com since 2010.