ARLINGTON -- The Rangers are planning to start Shelby Miller on Wednesday against the Pirates. But manager Chris Woodward admitted that the Rangers are mulling the possibility that Miller may be better suited in the bullpen.

Miller has struggled as a starter since returning from Tommy John surgery, going 1-2 with a 7.99 ERA in six starts for the Rangers. But he is averaging 94.2 mph with his fastball, the fourth highest on the staff, and that could go higher in short stints out of the bullpen.

“We are going to obviously talk about everything and see what makes the most sense,” Woodward said. “I can’t say that right now. It’s definitely an option. It’s not a slight on him [or] thinking he can’t start. He’s got really good stuff and it plays at 94-95 [mph], out of the bullpen it might be 95-96 [mph]. But obviously we’ll discuss that with him if it ever gets to that point.”

Miller has struggled with runners on base. Opponents are hitting .316 against him with a .500 on-base percentage and a .500 slugging percentage when there are runners on base. The Major League averages are .253/.333/.424.

With nobody on base, opponents are hitting .281 off Miller with a .339 on-base percentage and a .439 slugging percentage. While not dominant, that's closer to the Major League average of .245/.314/.433.

“It’s something he’ll have to deal with,” Woodward said. “Every pitcher has to deal with it, knowing how to pitch with runners on base. Limiting damage is critical, not allowing things to snowball. Everybody is going to give up hits with runners on base, I get it. But in his case, that’s been his biggest issue. If he figures that out, he’ll be fine.”

Miller has walked 20 batters, and 15 have come with runners on base.

“The walks are killing us as a staff with some of the guys who are struggling,” Woodward said.

If the Rangers decide to move Miller to the bullpen, their options for the rotation could be righty Ariel Jurado or Rule 5 rookie right-hander Kyle Dowdy.

“We all look at Shelby and there is so much potential there in any way from starting, or if we were going to make that decision and put him in the bullpen,” Woodward said. “He could be a power arm out of the bullpen.”

Guzman to begin rehab assignment

First baseman Ronald Guzman, who has been on the injured list with a strained right hamstring, is scheduled to begin a rehab assignment with Double-A Frisco on Saturday. Guzman will likely play 3-4 games for the RoughRiders either at first base or as the designated hitter and then join the Rangers on Thursday in Houston.

Danny Santana and Logan Forsythe have been filling in at first base during Guzman’s absence. The Rangers will have to make a decision when Guzman returns as to whether they are going to drop one of those two players from the roster, or go to a seven-man bullpen.

“I think we have taken it slow,” Woodward said. “That has nothing to do with it. Anytime it’s a soft tissue or muscle injury, you have to be careful. Especially a guy who has never had it. Just don’t want it to linger or have a repeat injury.”

Rangers beat

• The Rangers have confirmed that Drew Smyly will come off the injured list Sunday and pitch against the Blue Jays. He has been shut down since his last start on April 19 with fatigue in his left arm, but was cleared after throwing a simulated game Wednesday.

• Right-hander Seth Maness threw six scoreless innings for Triple-A Nashville on Thursday before allowing four runs in the seventh inning against Omaha. Maness has made 252 Major League relief appearances for the Cardinals in 2013-16 and the Royals in 2017. He was released by the Royals last season and the Rangers signed him after he made one start with High Point of the Atlantic League.

• Blue Jays third baseman Vladimir Guerrero Jr. is 20 years and 48 days old, making him the seventh-youngest player ever to suit up at Globe Life Park. He is the youngest since Jurickson Profar played here at 19 years and 223 days old in 2013.