OAKLAND -- Hearing the word "back" hasn't necessarily been music to Eric Chavez's ears during the past few months. After all, it has been back spasms that have kept him out of the A's lineup since he was placed on the disabled list last August.

But on Thursday, the word "back" was the reason behind contagious smiles in the A's clubhouse before their series finale against the Blue Jays -- and for good reason.

Chavez was back in the Oakland lineup in the designated hitter spot. The third baseman was activated for the day's game as the injured Frank Thomas heads to the DL with a strained right quadriceps.

The A's designated outfielder Jeff Fiorentino for assignment to clear a spot for Chavez on the 40-man roster. Chavez had been with the River Cats in Tucson over the weekend, and it was there he said he felt rejuvenated. Surprisingly, though, it wasn't his back that had been keeping him from feeling like himself.

"My back's felt good the whole time," he said. "It was my legs I was especially waiting on to catch up. And my shoulder's been a bit sore.

"That's what happens when you're not out there on the field every day."

The third baseman began the season on the DL as he continued to rehab from three off-season back surgeries over a 10-week span and was transferred to the 60-day DL on April 24.

Chavez, who was expected to play nine innings at third base for three consecutive days in Colorado Springs, said he called the A's trainers Wednesday afternoon to let them know he "didn't need three games in a row, to tell them I was ready."

Upon learning Thomas' injury was going to be more than a day-to-day problem, manager Bob Geren said "it worked out perfectly" to have Chavez ready to fill in for the Big Hurt.

"He said he felt good, and he was swinging the bat well," the skipper said before Thursday's game. "We talked to everyone involved, and here he can build up his innings at third the same way."

Geren said Chavez will be playing at third base for the first two games in Texas beginning Friday, and after that, he will be alternating between the DH spot and the hot corner. The six-time Gold Glove winner said having a batting cage at home helped him keep his swing in a consistent mode, and come Friday, it will be interesting to get a feel for a big league field for the first time this season.

"I should be, at least, an average third baseman," the 30-year-old said with a laugh. "I'm anxious to see how I go out there and respond to myself."

Anxious, yes. But nervous? No way.

"Maybe I'm too tired to be nervous," he said while cracking a half-smile.

Chavez had been batting .367 with two home runs through nine games with the River Cats and said the only time he felt any sense of nerves was during his first rehab start for the Triple-A team.

"That first game in Sacramento felt like something I had never experienced before, mostly because there were a lot of question marks," he said. "It was definitely the weirdest day for me as a professional."

Those days, however, have hopefully been put to rest. Chavez repeatedly insisted he "feels pretty good about the chances of staying healthy."

"I just want to play baseball," the Oakland fan favorite said. "I want to stay healthy; that's probably my main goal. I'm really not worried about expectations."

Geren made no secret that he was just as pleased to have the infielder back, as was every player surrounding the third baseman in the clubhouse.

"It's great to see him back," Geren said. "We're obviously going to miss Frank, but I think having anyone with [Chavez's] background is a big boost.

"A lot of the guys were excited to see him in the lineup this morning."

Among them was Thomas, who appeared frustrated but optimistic about his trip to the DL.

"It's a big-time strain in a scary spot, so it's got to calm down," the slugger said. "I'm happy Eric's back, though."

Thomas said he's hoping to start hitting again in 8-9 days, but in the meantime, the designated hitter will continue to load up on the ice.

While the news about the Big Hurt wasn't what Geren was hoping to hear, the A's got relatively good feedback about Ryan Sweeney, who left Wednesday's game after fouling a ball off his toe. An X-ray revealed no fracture or break, prompting a sigh of relief by the skipper.

"He's day-to-day," Geren said of the outfielder. "But aren't we all?"