The Royals expect Mazzaro to fit into the starting rotation next season. Marks, a Minor Leaguer, was assigned to Class A Wilmington.
Trading DeJesus came as no real surprise.
"He's a free agent at the end of the 2011 season and, because of that, we felt we had to try to make a deal that was a good baseball deal and brought us back some pitching depth for the future," Royals general manager Dayton Moore said.
DeJesus, however, was somewhat surprised.
"I had no clue it was coming," DeJesus said. "But the Royals wanted pitching and they got a pretty good pitcher -- and the other dude is a pretty good prospect. Going to Oakland, they've got great pitching. And if we can get some bats over there, I think it'll be an exciting year."
DeJesus, a left-handed batter, hit .318 in 91 games last season before he crashed into the center-field wall at Yankee Stadium on July 22. Four days later, he underwent surgery on his right thumb and was out for the rest of the season.
Mazzaro was 6-8 with a 4.27 ERA for the A's in 24 games, including 18 starts. He also pitched for Triple-A Sacramento, going 3-1 with a 3.13 ERA. Moore sees him joining a rotation that includes Zack Greinke, Luke Hochevar, Sean O'Sullivan and Kyle Davies.
"He's 24 and had decent success at the Major League level. He just needs to go out there every fifth day," Moore said. "He's a big, strong guy who our scouts feel can be a solid rotation pitcher for a number of years -- and we control him for five years."
That wasn't the case with DeJesus, who holds a $6 million contract after the Royals picked up his option near the end of the season.
"David was in the final year of his contract," Moore said. "He was a terrific performer for us, the most consistent player offensively and defensively. He's a terrific human being, great in the community, great in our clubhouse."
Asked if there was an effort to sign DeJesus to a multi-year extension, Moore said: "For what we're trying to do going forward, it just didn't fit at the present time."
Typically, DeJesus was optimistic as he looked ahead to joining the A's.
"It was like a shock because I was with Kansas City my whole career, so it's kind of like, 'Whoa!'" said DeJesus, who was the longest-tenured Royal. "But I know Coco [Crisp] and a lot of players on that team. Jack Cust, I grew up playing against -- and he's still there. But, you know, it's one of those things and that's the way the game is."
The addition of Mazzaro gives the Royals a probable fifth right-hander for the rotation, after Brian Bannister was let go earlier on Wednesday. Bannister opted for free agency, rather than accept an assignment to Omaha. But Moore pointed out that the Royals were looking for young right-handers because the majority of their emerging prospects are lefties.
Marks, 22, was in his first full season in the Minors last year with two Class A clubs. He was 3-12 with a 4.92 ERA in 20 starts for Kane County, but also had 119 strikeouts in 109 2/3 innings. Promoted to Stockton, he was 3-1 with a 4.58 ERA in five games.
"Justin Marks was a guy we liked out of the Draft two years ago out of the University of Louisville and we had a chance to scout him last year in the Midwest League," Moore said. "He's a big, strong left-hander [who] averages a strikeout an inning and someone who can fit in with the wave of young pitchers we have coming."
Along with DeJesus, the Royals' other big name who keeps surfacing in trade speculation is Greinke, the '09 Cy Young Award winner.
Although he wouldn't discuss the possibility of a Greinke deal, Moore said: "I know there's a lot being written about it nationally, and there's a lot of talk because of how limited the free-agent market [is] and the lack of depth in the rotation, but you just evaluate situations as they play out. We're not in a hurry to do anything with any of our players. We weren't shopping David."
Moore said no other deal was imminent.
"I don't anticipate anything happening real quick, unless someone just called and blew us away," he said.
Moore is awaiting Greinke's list of teams to which he would not accept a trade.
With DeJesus leaving, the Royals might look at free-agent outfielder Jeff Francoeur, who declined the Rangers' Minor League assignment. The Royals have had interest in the right-handed hitter, who was drafted by Atlanta when Moore was with the Braves.
"Obviously, there's a lot of history there and because of that, there's a lot of speculation -- and Jeff's a winning-type baseball player and brings a lot of energy," Moore said. "But it's like anything else, there's got to be a fit -- and it's been a few years since we've been together, but I've got a lot of respect for him and the way he plays the game."
Francoeur could fit into a platoon situation in the Royals' outfield.
With DeJesus' $6 million contract off the books, some payroll money is freed up. Moore noted that it gave him some "flexibility," but emphasized he wouldn't be signing long-term deals with players who would interfere with the timeline he envisions for young prospects coming up through the system.
DeJesus, 30, started last season slowly with a .257 average in his first 37 games, but was blazing at a .358 clip in his next 54 games when, quite literally, he hit the wall. Adept at any of the three outfield positions, he goes to the A's with a 241-game errorless streak intact. In '08, he was brilliant in left field and notched a career-best 13 assists.
His repaired thumb on his glove hand, he said, is healing nicely.
"It's feeling great, everything is going good," DeJesus said. "I don't start swinging for like another month, so I'm going to give it time to heal and make sure it's ready to go. But I feel like once I start going, it should be OK."
Primarily a leadoff batter for the Royals, he was never a big basestealer, but he ran the bases well. DeJesus also proved he could operate out of the No. 3 spot in the order quite effectively.
He first joined the Royals for 12 games in '03, then hit .287 as a rookie in '04 and was so impressive in 96 games that Kansas City writers voted him the club's Player of the Year. In '08, he knocked in a career-high 73 runs and hit .307 with 12 homers.
"There were a lot of emotions with David," Moore said. "He's a great kid and one of those guys you could always count on to have an upbeat, positive attitude every day. He always brings a lot of energy and positive influence to everything he does. He's a terrific person and you hate to see him leave, but as I said, it's the right thing for our baseball team today."
DeJesus said he appreciated Moore's comments.
"I'll miss Kansas City, but it's the new part of my career starting," he said.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.