OAKLAND -- The Angels' greatest area of depth going into the season was suddenly their thinnest department as they began a three-game series against the Athletics on Monday, with Vernon Wells and Ryan Langerhans being placed the disabled list, Torii Hunter still away from the team and an extra pitcher added to the roster after Sunday's 13-inning loss to the Padres.

The Angels learned Monday that Wells would undergo surgery to repair the ulnar collateral ligament in his right thumb, which he injured during a stolen base in the second inning of the series finale at Petco Park, and would be away from baseball activities for the next 8-10 weeks.

That means Wells, batting .244 with six homers and owed $63 million through the 2014 season, will be unavailable until at least early August. And it means that for now, at least until Hunter returns, the Angels' starting outfield will feature Mike Trout in left, Peter Bourjos in center and Mark Trumbo in right.

"Any time you lose a guy that you're counting on, who's shown some signs of maybe getting back close to his game, it's unfortunate," Angels manager Mike Scioscia said.

"Hopefully on the offensive side, when we start to get a little more production from 1 to 9, you can absorb maybe that one bat that's not in the lineup."

With Wells going under the knife -- his surgery is scheduled for Tuesday in Los Angeles, and will be performed by hand-and-wrist specialist Dr. Steven Shin -- and Langerhans also being placed on the 15-day DL with a separated right shoulder, the Angels called up outfielder Kole Calhoun and reliever Bobby Cassevah from Triple-A Salt Lake.

Reliever Michael Kohn (Tommy John surgery) was transferred to the 60-day disabled list to create a spot for Calhoun on the 40-man roster.

Langerhans, who was called up when Hunter was placed on the restricted list Monday, hurt his shoulder while tracking down a John Baker double in the bottom of the 11th Sunday, then had his arm in a sling postgame. The 32-year-old won't require surgery, but Scioscia said "it's tough to say" how long Langerhans will be out.

"I think these things kind of go at their own pace, kind of like hamstrings," Scioscia added. "It's a definite separation in his AC joint."

Over the weekend, Scioscia said he was hopeful that Hunter -- back in Texas to be with his 17-year-old son, who's defending sexual-assault charges -- would rejoin the team at some point on this road trip, which ends in Seattle on Sunday.

But that timeline is still a bit hazy.

"He has a lot of things that he's working through back home," Scioscia said. "As soon as he's comfortable to get back, he will. We don't have a definitive timeline."

Until Hunter returns, Calhoun -- capable of playing all three outfield spots and first base -- will be the fourth outfielder and Bourjos will get more playing time after starting just six of 22 games since Trout's April 28 callup. Cassevah, a sinker-baller, gives the Angels 14 relievers after their bullpen accounted for 6 2/3 innings in Sunday's 3-2 loss.

Wells, who was batting .348 in six games leading up to the injury, hurt himself while trying to pick himself back up after a pop-up slide in the top of the second that afternoon. He stayed on the bases and took his position in left field for the bottom half but was subbed out in the middle of the third.

"He felt it move, and he felt it popped out and popped right back in," Scioscia said. "It happens a lot of times where it's just something that you work through. But as he came off the field, next inning, he had no chance to swing the bat and no chance to play. We knew at that point that it was something significant."

Calhoun ready to prove himself in big leagues

OAKLAND -- At this time last year, outfielder Kole Calhoun was trying to make a name for himself in 'A' ball. Now, he's in the Major Leagues, having jumped three levels in 12 months.

Not bad.

"That's your No. 1 dream," Calhoun said, a big smile decorating his face from the visiting clubhouse at the Oakland Coliseum on Monday. "But it was more just taking it day by day, and going out and playing the game every day. The opportunities kept coming, just kept doing all right, and here I am."

Calhoun, a 24-year-old left-handed hitter, is here because the Angels are a bit depleted in their outfield, with Vernon Wells (right thumb surgery) and Ryan Langerhans (separated right shoulder) both going down Sunday, and Torii Hunter still on the restricted list.

He's also here because of how he's impressed in a short amount of time.

Taken in the eighth round of the 2010 Draft after four years at Arizona State University, Calhoun was the Angels' Minor League Player of the Year in his first full season of pro ball last year, batting .324 with 22 homers and 99 RBIs for Class A Inland Empire -- an affiliate for which he proved too advanced.

This year, after a key learning experience in the Venezuelan Winter League, he turned a lot of heads in Spring Training and adequately made the jump to Triple-A, batting .296 with five homers and 31 RBIs while playing mostly center field in 43 games for the Salt Lake Bees.

Now, he'll be the fourth outfielder until Hunter returns, and may stay even longer if the Angels go back to 13 pitchers by that point. The Angels like Calhoun's versatility -- he can play all three outfield spots and first base -- and his maturity at the plate, and they believe he may be ready to perform his current role in the Majors.

"I'm ready to play," said Calhoun, who was able to get eight family members to Oakland on short notice. "I'm anxious, for sure, to get out there and get in the batter's box and give it my all."

Bullpen making strides after rough start

OAKLAND -- The Angels' bullpen suffered its seventh loss in the series finale at Petco Park, but that doesn't diminish the vast improvements the relief corps has made over the past week.

Since last Monday, the Angels' much-maligned bullpen -- still without LaTroy Hawkins (broken pinkie) -- has given up just three earned runs in 20 2/3 innings, dropping their collective ERA from 4.70 to 4.02. That mark still ranks 21st in the Majors, but manager Mike Scioscia now has more stability in the back end, with closer Scott Downs remaining consistent, newcomer Ernesto Frieri providing a spark and former closer Jordan Walden seemingly bouncing back.

The Angels on Monday called up 26-year-old sinkerballer Bobby Cassevah, who had a 6.73 ERA and 1.92 WHIP in 12 Triple-A innings, giving them an extra arm after the bullpen accounted for 6 2/3 innings (and one unearned run) in Sunday's 3-2, 13-inning loss to the Padres.

The Angels may stay with 14 relievers until outfielder Torii Hunter returns from the restricted list.