OAKLAND -- A's skipper Bob Melvin has never implemented a six-man starting rotation in his managerial career. But it's something he's experimenting with -- at least for one turn before the All-Star break.

Righty Rich Harden, who has been sidelined with a strained right lat muscle all season after signing a one-year, $1.5 million contract in the offseason, is making his Oakland debut in Friday's series opener with the D-backs, and right-hander Brandon McCarthy (stress reaction in right shoulder) will return from the disabled list on Monday.

That leaves the A's rotation with an unconventional six arms for the time being. For the next week the starters will go: Harden on Friday, Josh Outman on Saturday, Gio Gonzalez on Sunday, McCarthy on Monday, Trevor Cahill on Tuesday and Guillermo Moscoso on Wednesday.

Whether that rotation sticks or not will depend on a few factors, one of those being how well Harden, who will be limited in his pitch count, performs in his first start of the season.

"Certainly the outing is going to be a little shorter, but you look for sharpness right away," Melvin said of Harden. "You look for that in rehab too. He's pitched well in rehab. He's another new guy for me that I've never managed before, so as far as the intricacies of what he does, I need to learn as well as everyone else I have."

"I'm just looking for a look in his eye, and he's always been a guy that likes to compete, and I'm looking forward to watching him compete."

Another factor that will play into Melvin's decision is the performance of Moscoso, who has filled in amply in the A's rotation. With the way Moscoso (2-4, 2.51 ERA) has performed to date, Melvin is in no rush to bump the righty from the rotation.

"He's still got another start, and until that changes he's still on my board a couple times more," Melvin said. "I don't go too much farther out than that."

Weeks the second baseman of the present

OAKLAND -- When the A's traded Mark Ellis -- the longest-tenured player on the team -- to the Rockies on Thursday, they made it clear that rookie Jemile Weeks is not only the future at second base, but the present, too.

With Weeks' development coming along quicker than many expected, his performance on the field has come as a pleasant surprise to manager Bob Melvin.

"He's been very consistent, which to an extent has surprised me," Melvin said. "Younger players have a tough time staying consistent. They'll go through hot streaks, cold streaks and then they'll have to deal with the fact that the first time in their career they're struggling some."

But that hasn't been the case with Weeks through his first 21 Major League games. The rookie, who has worked his way into the A's leadoff role as of late, was hitting .309 and reaching base at a .349 clip entering Friday's series opener against the D-backs. He has also swiped as many bags (six) as he has RBIs, and has seven multiple-hit games through his first three and a half weeks in the big leagues.

He has also flashed his glove countless time on defense, sporting a .973 fielding percentage while helping turn 15 double plays.

And Melvin believes Weeks is just scratching the surface of his capabilities, saying the switch-hitting rookie will only continue to grow as he learns pitchers and gets more reps.

"Certainly the numbers he's putting up right now and what he's doing for us energy-wise and so forth, he's certainly forecast that throughout his whole career," Melvin said. "He's only going to get better."

Worth noting

• Since righty Rich Harden's pitch count will be limited in his debut Friday, the A's recalled Fautino De Los Santos from Triple-A Sacramento to give the team added depth in the bullpen.

To make room for De Los Santos, who arrived at the A's clubhouse at 5:50 p.m., the team sent down Trystan Magnuson, who pitched 3 1/3 innings of relief on Thursday. The A's also designated southpaw Bobby Cramer for assignment.

• Rookie Chris Carter was in the starting lineup for the second straight day, this time getting the nod at first while Hideki Matsui reclaimed his designated hitter role. The start at first is Carter's first of the season, though he saw time there while the team was in New York.

• Melvin, who grew up in the Bay Area, is good friends with D-backs manager Kirk Gibson, and fondly remembers Gibson's storied home run for the Dodgers during the 1988 World Series.

"There are certain things in the game that you always remember where you were, and that was one of them," said Melvin, who recalls being at a bachelor party when an injured Gibson hit the walk-off shot against A's closer Dennis Eckersley in Game 1.

"Watching the at-bat unfold, I was thinking he had no chance. But knowing him, and knowing he's going to fight through that thing, the deeper the count went the better chance he had," Melvin said. "Pretty special moment in all of baseball."