HOUSTON -- Astros right-hander Lance McCullers threw a simulated game on Tuesday in West Palm Beach, Fla., the next step in his return from an injury that has sidelined him for six weeks.

McCullers (10-6, 3.93 ERA) was lost to a right forearm strain on Aug. 4 against the Los Angeles Dodgers. He has logged career highs in innings (126), wins (10), and strikeouts (138) for the Astros (95-56). Whenever he returns it will be in a bullpen capacity. McCullers has yet to appear in relief during the regular season, but he did work out of the bullpen twice last postseason, allowing two runs on four hits and three walks with 10 strikeouts over seven combined innings in Game 3 of the American League Division Series and Game 7 of the AL Championship Series.

McCullers threw 30 pitches against three different batters. He is scheduled to throw another simulated game this weekend, at which point a decision regarding his availability will be made.

"He's still a little erratic getting back on the mound but the fastball seems to be hovering in the low 90s, the breaking ball is there, the changeup," Astros manager A.J. Hinch said. "It was another good step forward.

"I don't want to promise too much too soon, but the hope is that he needs another outing and then we'll reevaluate once we get on the road what's next for him. We're running out of schedule, so we need to push him a little bit but it will all depend on how things go this weekend."

Left-hander Dallas Keuchel (11-10, 3.57 ERA) will start on Wednesday for the Astros in what amounts to be the rubber match of the three-game and season series with the Mariners. Keuchel is 8-2 with a 3.16 ERA over his last 18 starts and is 1-2 with a 4.08 ERA over four starts against the Mariners this season. For his career against Seattle, Keuchel is 9-7 with a 3.02 ERA over 19 appearances (18 starts).

Mariners manager Scott Servais did not name a starter for the series finale, indicating that he could elect to tab left-hander Roenis Elias (2-1, 3.18 ERA) or opt for the "opener" approach and utilize his bullpen with right-hander Casey Lawrence a candidate on the mound for Seattle.

Mariners right fielder Mitch Haniger represents one guarantee. Haniger has been a second-half stalwart for Seattle (83-68), starting every game since July 7 while missing only three games this season. Haniger, who earned his first All-Star Game nod this season, paces the Mariners in bWAR (5.7) and has taken off since being elevated from the third spot in the order to leadoff.

Entering Tuesday, Haniger was slashing .329/.388/.597 over the 35 games since his move up the lineup on Aug. 9. He finished 1-for-4 in the Mariners' 7-0 loss against the Astros, participating in his 148th game while increasing his plate appearances this season to 643. His previous career highs for games played (96) and plate appearances (410) were recorded last season, his first in Seattle. His productivity and reliability this season have not gone unnoticed.

"He's grinding through, he's really tired," Servais said of Haniger, who's hit a career-best 25 homers and is tied for the club lead with 89 RBIs. "He's had an unbelievable season and he ain't backing off the gas until it's over, and I certainly appreciate that. Nobody appreciates that stuff more than the manager and the coaching staff. He'll continue to grind; that's what he does."