SAN FRANCISCO -- The D-backs got some mixed injury news ahead of Tuesday's series opener against the Giants at Oracle Park, activating slugging first baseman Christian Walker from the 10-day injured list but losing outfielder Lourdes Gurriel Jr. to a left calf strain.

While the return of Walker is a welcome development for a D-backs team that entered Tuesday in the second NL Wild Card spot and in third place in the NL West, six games behind the first-place Dodgers, the corresponding move was less desirable -- but manager Torey Lovullo said Gurriel is not expected to be sidelined for long.

"We're hoping to get him back on the 11th day," the D-backs' skipper said. "We just felt like it was something that was going to linger for him, and if he continued to play, it might be a little bit of a challenge for him."

The earliest Gurriel could return to the active roster is Sept. 12, as his placement on the 10-day IL was backdated to Monday. The veteran outfielder had suffered some tightness in his left calf muscle on Sunday afternoon while trying to avoid a double play in the bottom of the sixth inning. As he ran to first, Gurriel started to pull up a bit and was noticeably limping.

Gurriel underwent imaging in San Francisco that revealed the strain, and he said he will return to Arizona to go through his rehab progression, which will begin with rest.

Even if he misses the minimum time, Gurriel's bat will be tough to replace. A steady contributor for much of the season, Gurriel had particularly been swinging it well of late, hitting .338 with a .976 OPS in his last 20 games.

"We don't choose when these moments happen," Gurriel said through interpreter Rolando Valles. "I've got to stay mentally strong, and I just want to keep the momentum going."

Fortunately for the D-backs, they'll have another big bat to slot into the lineup with Walker back for the first time since sustaining a left oblique strain on July 29. The two-time Gold Glove Award winner was hitting .254 with an .814 OPS before being sidelined for all of August, and his 23 homers are tied with Eugenio Su¨¢rez for second most on the team behind Ketel Marte's 30.

Walker did not go on a Minor League rehab assignment, and he said there would be a "progression" with his return to play as he rediscovers his timing at the plate and gets reaccustomed to the daily intensity of the big leagues. He was in Tuesday's lineup, batting cleanup as the first baseman.

"I think if we all went on rehab assignments until our timing felt perfect, it'd be like Spring Training, right?" Walker said. "We'd be playing for a month. It's a little bit of like, all right, you trust it. You feel good. You know you're letting good swings go. It's like, all right, it's time to get back in the fight."

The absence of Gurriel and the presence of both Walker and Josh Bell -- who hit .283 with an .809 OPS in 28 games since being acquired from the Marlins at the July 30 Trade Deadline -- gives Lovullo an opportunity to get creative at designated hitter.

Arizona has generally used the DH as a platoon this season, going with Joc Pederson against righties and Randal Grichuk against lefties. With a Giants left-hander on the mound on Tuesday, Grichuk instead started in left field, and Bell got the nod at DH.

Lovullo indicated that when facing a lefty starter, he wants to have both Walker and Bell in the lineup, with one playing first base and the other DHing. That setup could look different against right-handers, Lovullo said, with Pavin Smith likely to get opportunities.

"I'm going to be as creative as I possibly can with that DH spot," Lovullo said. "I don't want to miss at-bats for Walker, that's for sure, but I've got to be careful with his return-to-play progression."

Walker, who has dealt with oblique injuries before, knows that it can be a difficult injury to return from. He's confident in the progress he made while on the IL -- and ready to contribute to the D-backs' stretch run.

"Injuries are one of those things; it's just as much about trusting it coming back as it is being healed and being ready," Walker said. "I'm looking forward to getting into some high-intensity moments and see how it reacts."