From Chronicle Staff Writer Susan Slusser

It seems almost unthinkable, but the A’s just dealt away their best player, All-Star and MVP candidate Josh Donaldson. Oakland has traded Donaldson, a premiere defensive player as well as Oakland’s No. 3 hitter, to Toronto for infielder Brett Lawrie, shortstop Franklin Barreto and pitchers Kendall Graveman and Sean Nolin.

“I’m so shocked,” Donaldson said. “I just got off the phone with Billy Beane, and I guess they got an offer that they couldn’t resist. I’m definitely a little emotional about it. Oakland is my home.

“The most important thing is that Billy gave me an opportunity to be on that team and I am very grateful for that, and for all the fan support I’ve had. I’m so glad to have been a part of that. But at the end of the day, it’s a business, as much as it hurts emotionally. The guys in that clubhouse are my brothers, the coaches are my father figures.”

Beane, the A’s general manager, just said on a conference call that the A’s were adamant they would not deal Donaldson when Toronto called, but Blue Jays GM Alex Anthopoulos was persistent, and finally had enough parts in the deal to get it done.

Over the past two years, Donaldson’s WAR has been second only Mike Trout’s in baseball, and the A’s had him under team control for four more years. Beane said, though, that the A’s felt that after finishing 11 games behind the Angels and barely holding off the Mariners – and looking at losing free-agent shortstop Jed Lowrie, starter Jon Lester and reliever Luke Gregerson – they needed to make a move “that wasn’t timid and got us into a position to get better every day rather than one that was maybe starting to deteriorate.”

The A’s do have a major need at shortstop with Lowrie leave and with top prospect Addison Russell traded to the Cubs in July. But Barreto is 18 years old, he’s not even close to ready. He’s considered more of a second baseman than a shortstop by some scouts, but “he’s a hell of a hitter,” one scout just told me. Beane said that Barreto will play shortstop in the Oakland system and he said that the A’s believe Barreto to be one of the best prospects in the minors.

Lawrie is a third baseman, and one American League exec tells me he thinks Lawrie is, like Donaldson, Gold Glove caliber there, and an “explosive player – when he’s healthy.” That’s been the biggest issue the past year or two, as Lawrie has spent time on the DL Beane said that Lawrie is recovering from an oblique strain in Arizona and will begin working with the A’s training staff there and strength coach Mike Henriques. Lawrie has had three oblique strains in three seasons, as well as a broken finger that cost him time last year. A Canadian – so extra popular in Canada – Lawrie also can play second base, another position the A’s would like to upgrade; Beane said he’d immediately go to third, but there’s little doubt that should the A’s find another way to fill the third-base spot, Lawrie could move over to second.

Graveman is a potential fifth-starter type, one scout said, and Nolin, a left-hander with a plus-changeup, could be a very nice bullpen pickup, the exec said. I’m told Graveman is a classic A’s find – he’s a “good makeup” guy, which in scout-speak means he’s smart and doesn’t get himself in trouble, and he is a savvy pitcher, works backwards.

The rumblings started early Friday evening that Oakland had a big deal in the works – one source who called me first to say something big was brewing said Jeff Samardzija was getting dealt – but Donaldson appeared all but untouchable. He does turn 29 the week after next, and he was a super-two player this year, meaning he was going to arbitration early. After placing fourth in the MVP race in 2013 and eighth in 2014, along with making the All-Star team, Donaldson was going to start to get pricy.

Lawrie also is entering arbitration for the first time, but after three years of service time. He will turn 25 in January, so is four years younger than Donaldson.

Beane dismissed a question on the conference call about Donaldson tweeting that the A’s ownership has money but just doesn’t spend it by saying it was the first he’d even heard of it, so it had no bearing on the deal.

Donaldson said the one real positive he’s taking away from the deal is the fact that the Blue Jays are going all out to win – they recently signed catcher Russell Martin, too. Donaldson is a big fan of Blue Jays slugger Jose Bautista -he largely modeled his swing on Bautista’s.

He found out while playing video games at home – his phone started blowing up with texts, including one from his agent that said, “Blue Jays ?” Donaldson said, “I thought, ‘Uh-oh, I just got traded.”

“We’ve had so much success, it’s going to be difficult to leave, but when you’re with Oakland, you understand that this day is eventually going to happen,” Donaldson said.

For his teammates, though, it’s tough to see an everyday player – one who was on the field despite numerous injuries down the stretch – head elsewhere.

“It doesn’t make sense to me,” outfielder Josh Reddick said. “We traded our best player the past two years. It seems like we’re going into a rebuilding drive.

“It’s sad any time you see your best player go, not just as a player but as a person. And he’s our three-hole hitter and best defensive player. He was big for our team.”

It’s possible that Samardzija could still be dealt; the A’s need remains at shortstop, and 10 teams have expressed interest, including the Red Sox and the White Sox.

If Oakland doesn’t deal the right-hander, though, the rotation is one of the better ones in the league, with Samardzija, Sonny Gray, Scott Kazmir and presumably a combination of Graveman, Nolin, Drew Pomeranz or Jesse Chavez. And Jarrod Parker and A.J. Griffin are expected to return from Tommy John surgery around midseason.

To clear spots on the 40-man roster, the A’s designated right-handed pitcher Josh Lindblom and first baseman Kyle Blanks for assignment.