D.C. United captain Steve Birnbaum has been ruled out of Saturday’s match at New England because of a concussion while two others seem doubtful and another is back in the mix.

Birnbaum suffered the injury in a head-to-head collision in the late stages of a 2-0 away defeat Saturday against the New York Red Bulls. The fourth-year center back reported to RFK Stadium on Tuesday but remained inside while the team trained.

He must pass concussion protocol before the medical staff will allow him to resume workouts. Hence, there is no firm timetable for his return.

“He’s still a touch groggy,” Coach Ben Olsen said. “We will go through all the proper steps to get him back when the time is right.”

Birnbaum suffered two head injuries last year, but neither resulted in a concussion. In last weekend’s incident, he was stretchered off the field and taken to a New Jersey hospital for further evaluation.

Over the years, concussions have afflicted many United players. The length of their absences has varied, and in the case of several, ended their careers. That group includes Alecko Eskandrian, Josh Gros, Devon McTavish, Bryan Namoff and, most recently, Chris Rolfe, who is pursuing non-soccer opportunities after missing most of last season and being unable to resume workouts this preseason.

Also, Charlie Horton claimed in a recent lawsuit that the effects of a concussion, allegedly inflicted off the field by teammate Fabian Espindola, sidelined him for good.

As a player and now a coach, Olsen has witnessed many teammates, opponents and pupils feel the effects of concussions.

“You feel sick,” he said. “It’s no different than I felt when I was 12 and saw this stuff in sports. Unfortunately, it’s a part of it. We’ve all been around sports, you have these moments where you understand the force and danger these guys live with, especially for guys that are brave and are putting their heads in a lot of spots like Steve. He’ll be fine. We’ll make sure we jump through every hoop and get him back quickly, but more importantly, healthy.”

With Birnbaum sidelined this weekend, Kofi Opare or Sean Franklin is expected to partner with Bobby Boswell.

Meantime, midfielder Patrick Nyarko and defender-midfielder Nick DeLeon did not practice Tuesday. Nyarko missed the Red Bulls match with a hamstring injury and DeLeon came off early in the second half with a groin ailment.

Nyarko is “going to be a bit,” Olsen said, suggesting he, too, will miss Saturday’s game against the Revolution at Gillette Stadium.

DeLeon is scheduled to undergo an MRI exam. “He’s got some hernia symptoms, lower ab stuff that usually clears up pretty quickly,” Olsen said.

Sebastien Le Toux — who replaced DeLeon, who had started for Nyarko — is the prime option to start on the left flank. Lamar Neagle is also in the running.

Striker Patrick Mullins was fully engaged in training Tuesday and on track to return to the 18-man game-day roster this weekend, Olsen said. Mullins strained a hamstring March 18 against Columbus. Jose Guillermo Ortiz has started up front the past three matches.

“It’s not great when they go out, but when they come back in, it’s nice to see,” Olsen said. “It gives a little fresh energy to the group, as well.”

>> Three United-affiliated players were named to under-17 national team squads ahead of the CONCACAF championship, April 21-May 7 in Panama. Homegrown signing Chris Durkin (Glen Allen, Va.) made the U.S. roster, while academy members Allexon Saravia (Alexandria) and Alexis Cerritos (Silver Spring) received Salvadoran call-ups. Cerritos’s father, Ronald, is a former MLS and Salvadoran national team forward who coaches part-time in United’s academy.