This story was excerpted from Martín Gallegos’ A’s Beat newsletter. To read the full newsletter click here. And subscribe to get it regularly in your inbox.

OAKLAND — Monday was a rather big day for Mason Miller.

For the first time since landing on the injured list with a fractured left pinky finger on July 25, Miller was scheduled to throw a live batting practice session without a cast at the Coliseum.

Given the importance of the occasion and what it could mean for Miller’s potential return, A’s manager Mark Kotsay wanted to make sure he got an up-close look. What better way to do that than to actually step in the box?

When a pitcher throws a live batting practice, two or three different hitters on the team will typically stand in to take at-bats. As Miller took the mound for his session on Monday afternoon before the A’s opened a three-game series against the White Sox, however, it was the 48-year-old Kotsay who made a surprise appearance. He threw on a green and gold A’s batting helmet, strapped on a pair of batting gloves and walked up to the plate, harkening back to his playing days with Oakland from 2004-07 as part of a 17-year Major League career.

“Yes, I personally stepped in the box off Mason,” Kotsay said. “You spend 17 years in a batter’s box, you have a pretty good idea of what it’s supposed to look like.”

Miller, who leads the Majors with 306 pitches of 100 mph or faster this season, took it easy on his skipper by sitting in the low-to-mid 90s with his fastball. Kotsay saw five pitches from the All-Star rookie closer and swung at four, noting during his pregame media session that he did not swing and miss at any of Miller’s fastballs, and he even hit one in play down the right-field line.

“It was a little Ohtani-like,” Kotsay joked. “Kind of butt out when he was coming 2-2 on the front door two-seamer he likes to throw sometimes. If they would have been shifting me to pull, I would have had a double down the line. It’s always fun to get an assessment.”

Kotsay’s overall assessment of Miller’s stuff?

“It looked pretty good,” he said with a smile.

Miller’s session went the full scheduled 15 pitches. The 25-year-old right-hander was wearing a modified splint on his left hand under his glove and felt pain-free while squeezing it during PFPs (pitchers’ fielding practice), leading Kotsay to believe there is a strong chance Miller will be activated off the IL when eligible for Wednesday’s series finale against the White Sox.

“That was the biggest thing,” Kotsay said. “It was based on the pain tolerance for squeezing the glove. He’s more comfortable that he can get the glove squeezed to protect himself on a ball coming back at him. … We’ll see how he bounces back [on Tuesday] and make a determination. We could possibly activate him by the end of the series.”

Miller has emerged as the most electrifying closer in baseball this season, posting a 2.21 ERA in 34 appearances with 15 saves. The flamethrowing A’s rookie has struck out nearly half of his batters faced, leading Major League relievers with 15.49 K/9 (70 strikeouts in 40 2/3 innings).