Jay is the kind of player McGwire likes to call "low-maintenance."
"He does his work, and he's gone," said McGwire, the Cardinals' hitting coach. "That's a beautiful thing.
"There's nothing better for a hitting coach [than] to have a hitter be his own hitting coach. He just understands himself so well."
Jay's hard work has him batting .383 (23-for-60) since Aug. 2, but there have been both ups and downs for the St. Louis center fielder this season.
After batting .343 through his first 27 games of the season, Jay was sidelined from mid-May to late-June with a separated right shoulder. He hit just .213 in 24 games following his stint on the disabled list, and by Aug. 1, his average had dropped to .283 -- the lowest it had been since the second game of the year.
"I feel like I've got my timing back," Jay said. "I'm just going out there and trying to keep things simple. One at-bat at a time, one pitch at a time."
During Jay's difficult stretch at the plate, he was asked frequently whether the lagging effects from his shoulder injury were affecting his swing. Jay's swing starts with a dramatic load of his hands, as he cuts them down from his shoulder and back before extending them out to the ball.
As Cardinals manager Mike Matheny put it, Jay's swing has "a lot of moving parts."
At the time, Jay said everything felt in sync, and he maintains that nothing has changed for him. Balls are just finding holes lately. McGwire backed Jay, and said that it'd be difficult for any player with any type of swing to get back into a groove after sitting for a month.
"There [were] a lot of start-stops. It's really tough, especially for a young guy, to get going and then stop," McGwire said. "He dealt with it and understood it. Everybody goes through these things. It's how you come about it and reach the other side."
Now on the other side of that slump, Jay has been filling a void at the top of the Cards' lineup. Shortstop Rafael Furcal has handled the leadoff spot for a majority of the season, but back stiffness began limiting his production in late July.
Jay was called on to pace the lineup from the top, and he has hit leadoff in 11 of the Cardinals' past 12 games.
"It's not something everybody can do -- hold that leadoff position," Matheny said. "He's done a very nice job of kind of changing his mentality and what he needs to do. He's just got a nice rhythm right now at the plate."
While Jay's bat has gone hot and cold, his glove has been as steady as any center fielder in the league. Jay hasn't made an error in 76 games in center, and his over-the-shoulder grabs while racing toward the wall have become a routine sight for Busch Stadium crowds.
"People are amazed at all his catches," McGwire said. "We're not, because we've seen it [before]."
"He's just got great instincts for the ball off the bat," Matheny said. "He's making our whole outfield better."
As he does in the cage, Jay said he also makes the most of batting practice as he roams the outfield while his teammates hit. That's the time for Jay to study how the ball is flying that day, and to practice his routes to the warning track.
Those dry runs translate over to game situations, where Jay is also quarterbacking the rest of the outfield. When positioning both himself and his corner outfielders, Jay said he takes into account how catcher Yadier Molina is calling a game, what his pitchers are trying to do and how opposing hitters combat those game plans.
"You just have to know how to play guys, know what the pitcher's trying to do against that hitter and being able to work on the first jump," right fielder Carlos Beltran said. "I think for being in the league for a couple years, he's getting an idea of where to play guys, so I think he's doing a great job."
Both at the plate and in center, the 27-year-old Jay has learned to be his own coach, which has paid dividends for St. Louis this season.
"When I'm in the outfield or in the cage, I always say that everything I do on the field is with a purpose," Jay said. "I'm always trying to work hard to get better every day.
"I want to be a complete player."
Mike Still is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.