Chapman, who routinely throws his fastball more than 100 mph, struggled with his velocity and command in Monday's 14-inning game against the Pittsburgh Pirates. The All-Star threw seven of his 23 pitches for strikes and averaged a season-low 94.4 mph on his fastball, according to ESPN Stats & Info.
Chapman walked three of the five batters he faced in the 10th inning and had to be taken out during an inning for the first time this season. Afterward, he was examined and diagnosed with a tired shoulder.
Chapman moved into the closer's role this season for the first time in his career. He has made a career-high 64 appearances, converting 35 of 40 saves with a 1.60 ERA.
"We're lucky we got him to this point," Baker said. "When you throw 100 mph, you're bound to tweak something sooner or later. He's at a plateau he's never reached before. This is another step in learning how to be a closer."
Chapman set a club record when he didn't allow an earned run in his first 24 appearances of the season. He moved into the closer's role on May 20 after Baker gave him time to ease into late-inning pressure situations.
Chapman had a club-record streak of 27 consecutive saves broken on Friday when he gave up three runs in a 5-3 loss to Houston. His fastball had its usual zip in that game.
The closer said Tuesday that he's felt no pain in the shoulder.
"It is a little fatigued," Chapman said, with a trainer translating. "I started feeling it a couple of games back. I just wasn't myself. It's nothing major. There's nothing wrong. I just feel weak.
"I think in five days or a week I should be ready."
Jonathan Broxton, acquired from Kansas City at the trade deadline, could take over the role while Chapman rests.
Baker said the club is watching Chapman closely as he learns how to pace himself in workouts between appearances.
"We monitored him as closely as we monitor anybody," Baker said. "He's still learning the role and how to handle the workload off the field. You don't want to wear yourself out training, but you want to train some. The guy works hard."
Information from The Associated Press was used in this report.