Yet, that unofficial title, ironically enough, belongs to Bobby Crosby.
"I mostly talk to Nomar about hitting," Pennington said Wednesday before the third game of a four-game series against the Rangers at Oakland-Alameda County Coliseum. "Bobby is the one I go to when it comes to what to do out in the field, positioning, footwork, things like that. We talk all the time."
The irony, of course, lies in Crosby's history with the club. He had been Oakland's starting shortstop since the start of the 2004 season, after which he was named the American League Rookie of the Year, but he lost his job when the A's signed Cabrera this spring, and he was publicly peeved when Pennington was handed the job after the Cabrera trade.
At the same time, Crosby went out of his way to say that his feelings had nothing to do with Pennington, who was the team's first-round pick in the 2005 First-Year Player Draft.
And according to Pennington, Crosby -- a.k.a. "Bones" -- has continued to go out of his way to offer advice, guidance and support.
"Bones has helped me more than anyone," Pennington said. "From my first Spring Training with the team, through this Spring Training, to when I got called up [last season] and all the way up to today -- he's always been there for me."
Relayed those comments, Crosby shrugged off the suggestion that working with his replacement is unusual or potentially awkward in any way.
"The guy's a teammate," Crosby said. "That's what teammates are supposed to do, help each other out. It doesn't matter what the situation is. It's just what you do."
Pennington played more games at second base than at shortstop during a 36-game stint with the big league club last season, but what he's done since taking over at shortstop suggests that he could be the team's long-term solution at the position.
"He's gotten a lot better since the first time he came up," Crosby said. "He's a lot smoother."
A former Texas A&M star, Pennington batted .320 over his first 14 games, and since coming out of a 20-game funk in which he batted .167, he was batting .386 (22-for-57 with eight multiple-hit games, 12 runs, five doubles, two triples, two home runs and a .470 on-base percentage over his past 16 games entering Wednesday.
Batting .284 overall, Pennington also has provided strong defense up the middle, with two errors in 213 total chances in 49 games for a fielding percentage of .991.
When asked to list Pennington's strengths, Garciaparra, Crosby and A's manager Bob Geren put his throwing arm at or near the top of the list.
"He's made a lot of plays this year because of his arm strength," Geren said.
Asked to list the areas in which Pennington needs to improve, Garciaparra mentioned footwork around the bag. Pennington didn't disagree.
Nor did he dispute Crosby's claim: "He has good actions, but he's just a little too quick sometimes."
"I have times when I get a little spastic," Pennington said with a laugh. "I'll rush things and speed things up, and that's where you get inconsistency. It sounds strange, because I've always played fast, but up here you have to try to play as slow as possible."
Garciaparra said Pennington also has to grow into the role as the infield's "backbone."
"As the shortstop, you have to kind of control everything out there," Garciaparra said. "He doesn't have that yet, but that comes with time and experience. He definitely has the ability and the tools. He doesn't look out of place at all."
If he does, he might hear about it from his unlikely but more-than-willing mentor.
"If I see something that can help him, I'll definitely mention it," Crosby said. "Any help I can give him, I'll give it."