ST. PETERSBURG, Fla. — Brock Holt has played six games on his injury rehabilitation assignment with Triple A Pawtucket, going 8 of 19 with four walks.

In his last two games, Holt reached base eight times, scored twice, and drove in two runs.

But when the struggling Red Sox faced the Tampa Bay Rays at Tropicana Field on Monday night, Holt remained on the disabled list and is not expected back until Friday, if then.

Holt has been out since May 19 with a concussion, his second in 21 months. Deciding when he should return to the majors is not like other injuries. A concussion does not heal like a muscle strain.

“This is a very unique one. We talked a lot about his form of a concussion . . . I wish I could say this is a straight-line end result from a time standpoint. But it’s not,’’ manager John Farrell said.

Michael Collins, a head trauma specialist at the University of Pittsburgh Medical Center, has coordinated Holt’s care through both concussions. Collins encouraged Holt to start a rehab assignment, saying the activity could help him overcome any lingering symptoms.

“The symptoms aren’t going to go away just by sitting around. This is a matter of getting active and being aggressive with it,’’ Farrell said. “That’s the recommendation of the medical staff.’’

But some issues are lingering, including what was described to Farrell as “disorientation.’’ After talking with Holt, the decision was made for him to play for Pawtucket on Tuesday and Wednesday and reassess from there.

“You would expect that he has a loud voice in this in how he’s feeling and that’s what being expressed right now,’’ Farrell said.

Four-star lineup?

Online voting for the All-Star team ends Thursday at 11:59 p.m. Based on the latest vote totals. Xander Bogaerts, David Ortiz, Jackie Bradley Jr., and Mookie Betts are in line to be selected as starters.

Bogaerts has a 1.3-million vote lead on Kansas City’s Alcides Escobar. Ortiz is up by 2.2 million on Edwin Encarnacion of the Blue Jays.

In the outfield, Mike Trout of the Angels (2,972,582) has a healthy lead on Bradley (2,184,884) and Betts (1,915,637). But Kansas City’s Lorenzo Cain (1,821,746) and Baltimore’s Mark Trumbo (1,801,969) are close behind Betts.

Miller added

The Red Sox added infielder Mike Miller to the major league roster and optioned righthander William Cuevas to Triple A Pawtucket.

Miller came in to play second base in the eighth inning then grounded out to shortstop in the ninth of a 13-7 loss.

Miller, 26, was a ninth-round draft pick in 2012. He started the season with Double A Portland and moved to Pawtucket quickly. In 52 minor league games this season, he has hit .251 with a .617 OPS. Miller does not have a home run in 183 at-bats this season and is a career .267 hitter.

Miller did not take part in major league spring training but was called up for four games and went 0 for 3. He has played primarily second base for Pawtucket with seven starts at third base and six at shortstop.

“Dream come true, wasn’t expecting it at all,’’ said Miller said, who replaced Dustin Pedroia at second in the eighth inning and grounded out in his only at-bat.

Miller learned of his promotion in an unusual fashion Sunday. While Pawtucket was on a bus ride to Rochester, N.Y., he received an e-mail from manager Kevin Boles, who was a few rows in front of him.

“He didn’t want to make a big commotion on the bus. He told me to check my e-mail, keep it quiet, and don’t show a lot of emotion right now,’’ Miller said.

Miller texted his wife and parents and flew to Tampa early Monday morning. He had several family members at the game.

Cuevas was called up Saturday and pitched in two games, throwing 2? scoreless innings with two hits, four walks, and three strikeouts.

“In the event we make some moves late, [we] don’t want to be shorthanded. It’s been our intent to get back to a four-man bench,’’ Farrell said.

Workman works out

Brandon Workman was on the field at 3:15 p.m. throwing in the bullpen, with Farrell and pitching coach Carl Willis watching. The 27-year-old righthander is coming back from Tommy John surgery and could soon go out on a rehab assignment. He remains intent on pitching in the majors this season. He is just over a year removed from surgery.

“That’s been my goal from the time I got surgery through today,’’ he said. “I’ve been working, trying to get myself in a position where I can come back sometime this year and be a contributing part.’’

Workman was shut down this month because of arm fatigue and to correct some soreness in his lat muscle. It was not related to his elbow.

“Typical with the Tommy John recovery, there’s going to be some ups and downs with the arm strength as you go through those rehab outings,’’ Farrell said.

The plan would be for Workman to start with Single A Lowell then move up to Triple A Pawtucket.

Workman has not appeared in a major league game since Sept. 18, 2014.

He played a big role pitching out of the bullpen during the 2013 postseason.

Buchholz hangs in

Farrell indicated that Clay Buchholz (3-8, 5.90) would make his next start. Of the 61 American League pitchers with at least 60 innings, only Detroit’s Anibal Sanchez (6.06) and Baltimore’s Mike Wright (6.12) and Ubaldo Jimenez (6.97) have higher ERAs. Wright is now in the minors . . . Ryan Hanigan, also on a rehab assignment with Pawtucket, had Monday off and is scheduled to catch nine innings on Tuesday and Wednesday. His status will be determined on Thursday . . . Righthanded reliever Carson Smith, who had Tommy John surgery last month, was in uniform for the game. He has been rehabbing in Fort Myers . . . Bryce Brentz is 7 of 15 with three extra-base hits since being called up . . . The Red Sox dropped to are 1-3 against the Rays this season . . . When Ortiz walked with the bases loaded in the fifth inning, it gave him 1,703 RBIs, 24th all-time. He needs two more to pass Frank Thomas.

Peter Abraham can be reached at pabraham@globe.com. Follow him on Twitter @PeteAbe.