Jackson's move to the DL was made retroactive to Thursday, and he said he hopes to be activated by July 16, the first day he's eligible to return.
"It's frustrating," Jackson said. "After missing so much time last year and missing 18 days from this year already, now this. But I'm not going to let it drag me down. I'm going to rehab and try to get back healthy as possible."
Jackson said he first strained his hamstring on June 17 at Chicago's Wrigley Field -- playing his third game with the A's after being traded by the Diamondbacks -- and aggravated the injury the next day in St. Louis. Jackson missed 18 games for Arizona -- spending time on the DL with a strained right hamstring from April 18-May 8 -- but he said the injury occurred in a different spot this time.
"I can swing the bat, but it's the running that gets me," Jackson said. "We'll do treatment for the next six or seven days, and hopefully by the All-Star break I can get out and maybe go down to Rookie [League] ball and play some games down there."
Mortensen, 25, made his 2010 debut in place of injured lefty Dallas Braden on Saturday, allowing four runs (three earned) over six innings, while striking out seven and walking two. Mortensen is 10-2 with a 3.58 ERA in 15 Triple-A starts this season and was recently named to the Pacific Coast League All-Star team.
Due to the next week's All-Star break and an off-day on Thursday, the A's can afford to go with four pitchers until the Midsummer Classic. Oakland is hoping Braden and fellow lefty Brett Anderson can return to the rotation shortly after the break.
Carson joins the A's for his third stint this year and entered Monday having gone 1-for-9 in four games so far. In 51 games with the River Cats, Carson was batting .301 with nine home runs and 29 RBIs.
"It's good to be back," Carson said.
Watson, 31, hasn't played in the Majors since 2005, when he batted .188 in 48 at-bats with Oakland. Watson remained with the A's Triple-A affiliate in '06 before heading to Japan and playing for manager Bobby Valentine and the Chiba Lotte Marines in '07.
Watson returned to America in 2008, playing for the Blue Jays' Triple-A affiliate in Syracuse, N.Y., before signing with South Korea's Doosan Bears in '09. He also played in the Mets farm system in '09 and with an independent league team in his hometown of Lancaster, Pa.
The A's signed Watson on May 28, and he hit .313 with eight home runs and 23 RBIs in 28 games with Sacramento.
"I always like the perseverance of a young man that keeps trying to get his opportunity in the big leagues and to keep that dream alive," said Oakland manager Bob Geren.
Neither Carson nor Watson was in the starting lineup against the Yankees on Monday, but Geren said Carson could be in the lineup on Tuesday against left-hander CC Sabathia.
Doug Miller is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.