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6/10/2014 3:09 A.M. ET

Skaggs heads to DL with strained right hamstring

ANAHEIM -- Angels lefty Tyler Skaggs has been scratched from his Tuesday start against the A's because of a strained right hamstring, prompting Hector Santiago to come back from Triple-A Salt Lake to take his place.

Angels manager Mike Scioscia said Skaggs, who will be placed on the 15-day disabled list, "felt it a little bit" during his Thursday outing in Houston and "came out of the start a little bit sore."

The 22-year-old Skaggs -- 4-4 with a 4.34 ERA in 12 starts -- can be activated as early as June 21, which would have him missing two or three starts. And Scioscia doesn't anticipate this being a prolonged injury.

"We're doing this more as a precaution to make sure it gets behind him," Scioscia said. "Hamstrings, they have a life of their own. You never know. But we don't anticipate it being longer than [the extent of the DL stint]."

Skaggs said he didn't really feel like he had a hamstring injury during his start, but once he got off the plane later that night "it was super stiff," so he didn't even throw his between-starts bullpen session.

"I don't know the exact play where it happened," Skaggs said. "I think it was one of those things where it kind of progressively got worse during the game, but I didn't really feel it because I was so into the game. It's kind of like a freak thing, because I didn't even notice it until I got off the plane."

Skaggs called the injury a "minor setback" and expressed regret that he would missing Tuesday's start against the A's "because they definitely left a sour taste in my mouth the last game I pitched against them. Looking forward to pitching against them again, though."

Santiago was optioned to the Minor Leagues shortly after going 0-6 with a 5.19 ERA in his first seven starts with the Angels. The 26-year-old left-hander had a 6.43 ERA, a 2.14 WHIP and a 1.29 strikeouts-to-walks ratio in the three starts that spanned 14 innings during his time in the Pacific Coast League. Santiago last pitched Thursday, giving up six runs (four earned) on 11 hits in six innings, and will take the mound on his normal four days' rest.

"He's making progress with his command," Scioscia said of Santiago. "He feels much better about what he needs to do on the mound, so hopefully he'll bring it into the game tomorrow."

Freese: Angels' focus should be simple

ANAHEIM -- The Angels are trying their best to not make this week about the A's, regardless of closer Ernesto Frieri's colorful quote on Sunday.

They were 1-5 against the A's heading into the start of a three-game series at Angel Stadium, and trailed them by 4 1/2 games in the American League West. But third baseman David Freese, who was in his fair share of division races during his days in St. Louis, doesn't believe their fate will ultimately rest on their success or failure against the team that currently sports the best record in the AL.

"I think we feel that we just need to go out and win," Freese said. "This game's set up to play out on how many wins and losses you have to get in the postseason. It doesn't matter who you beat. Sure, it helps to take care of your division, but in the end, it matters how many wins you have at the end of the season, not who you beat."

Freese's philosophy comes a day after Frieri laid down the gauntlet, in regard to the division leaders.

"We're going to beat 'em," Frieri said of the A's after picking up his 10th save in Sunday's 4-2 win over the White Sox. "I hate to say this, but they've had a little bit of extra luck. If you pay attention to every play, it's stupid how the games go their way. But it's going to turn."

The A's entered Monday leading the AL in winning percentage (.619) and easily leading the Majors in run-differential (plus-128), while ranking second in starting-pitcher ERA (3.06), third in relief-pitcher WHIP (1.09), first in runs per game (5.19), third in OPS (.757), tied for third in defensive runs saved (22) and second in wins above replacement (13.8).

In other words, they're good.

"All three aspects, they do it well, and it shows," Freese said. "It's the reason why right now they're one of the best teams in the game."

But Freese believes the Angels -- sixth in the Majors in winning percentage, fifth in run-differential, 11th in starters' ERA, tied for 20th in reliever WHIP, fourth in runs per game, fifth in OPS, 11th in DRS and first in WAR -- are better-served keeping their focus internal.

Cliche as that may sound.

"You try to make it that simple," Freese said. "Everybody else is going to escalate it, but I think Oakland over there and the Angels over here, you just see it as a game that you want to go out and win. The goal during these six months is to win every series.

"The schedule is set up in September that you basically play your division and that's how it escalates a little bit more."

Aybar has bounced back defensively

ANAHEIM -- Erick Aybar is playing in his first season since turning 30, which isn't historically a time when players see their defensive ratings spike. But the Angels' long-time shortstop has made tremendous strides from 2013 to '14.

Last year, Aybar's ultimate zone rating -- a statistic that attempts to quantify how many runs a player saved or gave up through his defense -- plummeted to a career-low -6.6, as calculated by FanGraphs.com.

This year, his UZR is up to 3.5, which is a lot closer to his career rate at shortstop (6.6) and is on pace to be his highest mark since he recorded a 5.1 in 2009.

Aybar doesn't pay any attention to advanced defensive metrics, but he has noticed a difference.

"I feel more confident," Aybar said in Spanish. "More comfortable."

The reason, Aybar added, is twofold.

The veteran switch-hitter put in a lot of extra work over the offseason, training at a Dominican baseball academy in his native Bani for five hours per day, virtually every day, from the start of November until the end of January.

And Aybar's left heel, the one that sent him to the disabled list for three weeks last April, is finally healthy. Aybar returned on April 30 last season, but revealed on Monday that the heel bothered him throughout the 2013 campaign -- and didn't go away until he took the month of October off.

"I couldn't put pressure on it, or have that quick burst with my first step to go get to balls," Aybar said. "I was scared of getting hurt again. But this year, thankfully, my legs are healthy, my arm feels good. Everything's good so far."

Worth noting

• The Angels have yet to come to an agreement with their two top picks from the 2014 First-Year Player Draft, left-hander Sean Newcomb (15th overall) and right-hander Joe Gatto (53rd).

"We're working on it," scouting director Ric Wilson said. The slot values for Newcomb and Gatto are $2,475,600 and $1,050,600, respectively.

• The Angels may go all year without a lefty in their bullpen, and Angels manager Mike Scioscia believes that can be combated with a dominant right-handed bullpen, like he had with Scot Shields, Francisco Rodriguez and Troy Percival from 2002-04.

"We're hopefully evolving to that, but we're not there yet," Scioscia said.

Alden Gonzalez is a reporter for MLB.com. Read his blog, Gonzo and "The Show", and follow him on Twitter @Alden_Gonzalez. This story was not subject to the approval of Major League Baseball or its clubs.