He's the one-time catcher who was part of a package the A's got from the Chicago Cubs in the July 2008 deal that sent Chad Gaudin and Rich Harden to the Windy City. He made a big league cameo in 2010, spent all of '11 in the Minors, and then in '12, when he was being converted full time to third base and was in the midst of his second demotion of the year to Triple-A Sacramento, he accepted the challenge of A's player personnel director Billy Owen to play as well in the big leagues as he did in the Minors.
Asked to fill the hole at third base in August that season when Brandon Inge went on the disabled list, he hit .290 with 11 doubles and eight home runs in the final six weeks while the A's went on a season-ending 27-11 run that included a season-ending series sweep of Texas that allowed them to edge the Rangers by one game for the division title.
In his first full big league season, he was the A's MVP a year ago, finishing fourth in the AL MVP voting, even if he didn't merit enough support from fans, players or All-Star manager Jim Leyland to get invited to the All-Star game a year ago.
This year, however, it appears he has arrived. When the first vote totals for the AL All-Star team were announced on Monday, Donaldson was the leader at third base.
And he added another piece of campaign material to his portfolio on Wednesday night.
With Detroit clinging to a 1-0 lead, A's on first and third and Tigers closer Joe Nathan on the mound, Donaldson turned on the first pitch Nathan threw him -- a slider -- and drove it over the left-field fence for a walk-off home run, the second of his career, and the final shot in the fourth walk-off game in the Majors on Wednesday night.
Ryan Howard delivered a two-out three-run home run off Boone Logan in Philadelphia to lift the Phillies to a 6-3 win over the Rockies. Pitcher Juan Carlos Oviedo's errant throw allowed Kevin Pillar to score and lifted the Blue Jays to a 3-2 win over the Rays. Moises Sierra singled home Leury Garcia to lift the White Sox to a 3-2 win over Cleveland.
And then there was Donaldson, who capped off a bottom of the ninth that began with Tigers starter Anibal Sanchez getting a groundout from Alberto Callaspo. But Sanchez gave up a double to Coco Crisp, and gave way to Nathan. John Jaso greeted him with a single on a soft single that Tigers third baseman Nick Castellanos admitted was catchable, and Donaldson hit the next pitch where it was uncatchable to anyone in uniform.
He stood at home plate, not admiring the drive, but rather watching to make sure it stayed fair.
"The Tigers hit three or four that went foul [earlier in the game]," he said. "If it was not going to stay fair, I didn't have to waste my breath."
Then Donaldson exhaled, finally.
"It's tough as a hitter," he said of the situation. "You get over-excited some times. The music is going. And everybody is jumping up and down, and you want to come through.
"I was trying to take a deep breath and concentrate on the type of pitch [Nathan would throw]. You want to calm your nerves."
He paused and smiled, adding, "not that I've been able to do that," he said with a smile.
What he has been able to do, however, is make himself one of the game's elite players. He's tied for the AL lead with 43 runs scored, second in the league with 12 go-ahead RBIs, fifth in the league with 13 home runs, and sixth in the league with 41 RBIs.
He is the kind of guy who made the A's feel they were very much alive when he came up in the bottom of the ninth, even if they had managed only three baserunners in the first eight innings against Sanchez, only one of whom got past first base.
"I don't think anybody [on the A's] thought we'd come up short," said manager Bob Melvin. "As soon as Coco got on, we felt we were going to win the game. That's something we've been good about, feeling like we are going to win, until that final out."
Hard for the A's to feel any other way, especially on Wednesday night.
They, after all, were set up for Donaldson to get one last at-bat.
And he's the guy they have come to count on.