D.C. United midfielder Zoltan Stieber is denied by Orlando City’s Joe Bendik during Aug. 12 match at Audi Field. (Nick Wass/Associated Press)

The MLS season is a long slog and, because of numerous variables, players enter and exit the lineup regularly.

D.C. United’s formation change two weeks ago cost Zoltan Stieber his starting job, but a teammate’s injury this past weekend is likely to usher him back onto the field for two critical home matches. And next week, he will have to step aside again while on duty with the Hungarian national team.

In his first full season in Washington, the 29-year-old midfielder is tied for third on the team with five goals and third in assists with seven. He has played on both flanks and in the middle, but with Yamil Asad recovering from an ankle ailment, Stieber is Ben Olsen’s top option on the left wing Wednesday against the Philadelphia Union at Audi Field.

He was Olsen’s first sub the past three games, totaling 92 minutes and scoring an insurance goal against New England on Aug. 19.

“I don’t think anybody [likes] being on the bench,” Olsen said. “That’s part of being a pro athlete and wanting to be on the field. It’s partly why these guys can come in and do the job because they understand they are capable players and they have an argument they can be in the starting lineup.”

Stieber had started 14 consecutive matches when Olsen altered the formation to include two defensive midfielders instead of one. That meant one of the attackers would have to sit. Stieber was the odd man out, relegated to a super-sub of sorts and serve as a fresh winger in the second half.

“At first, I was disappointed,” he said, “but I respect the decision and try to do my best off the bench.”

Until Sunday, when a five-game unbeaten streak ended with a 1-0 defeat to the New York Red Bulls, United was humming along with Asad and Paul Arriola flanking playmaker Luciano Acosta and Wayne Rooney up front. Russell Canouse and Junior Moreno formed a smart, effective partnership in deeper midfield positions.

At Red Bull Arena, Asad injured his left ankle late in the first half on a hard, clean open-field tackle by Kemar Lawrence. Asad, who shares the team lead in goals with nine, tried to continue after intermission but departed in the 49th minute.

Asked about Lawrence’s challenge, Olsen said he did not have any lingering issues. “Initially, I thought it was nasty,” he said. “The players didn’t react in a crazy way. I haven’t heard anything [from the team]. I didn’t look at it [on video] again. It was a physical game.”

Stieber entered but, despite United’s quality chances to pull even, the Red Bulls prevailed.

With a heavy schedule the last two months, Olsen said he believes he is equipped with adequate depth to compensate for injuries, suspensions and international call-ups. The mentality of the reserves, such as Stieber, Darren Mattocks (nine goals) and Ulises Segura, is critical to sustaining good vibes.

“The guys who have come off the bench, even though not happy about it, have come in and been selfless and understand those moments count for them and the team,” Olsen said. “There are so many periods of ebbs and flows throughout the season. We tell the guys to make sure they stay checked in. Be upset but don’t remove yourself because, just like that, you are back in the lineup and then you stay in for four games.”

With Asad perhaps missing Sunday’s game against first-place Atlanta United as well, Stieber is on the spot.

“Things change very quickly from one game to the next game, so you have to be mentally there,” Stieber said. “Even if you don’t start, you have to stay focused and be there for the team. Ben needs every single guy for these games and for us to achieve our goals.”

After the Atlanta match, Stieber and other D.C. players will join their respective national teams for about a week. While away to face Finland and Greece, Stieber will miss the Sept. 8 away match against New York City FC and the Sept. 12 home game against Minnesota United.

Olsen said his message to anyone who finds themselves on the bench is: “Next man up. Now it’s your turn.”

United (7-10-6) is in eighth place in an Eastern Conference race that will send six teams to the playoffs but has played at least two fewer matches than the other contenders. It will host nine of the last 11 at Audi Field. A victory Wednesday would lift United over New England and into seventh, three points behind Montreal and six back of Philadelphia.

“If we want to get into the playoffs, these are the kinds of games we’ve got to win,” Stieber said. “We should’ve gotten something out of the game in New York. Now we have to show we can be strong at home and get the three points.”

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D.C. United vs. Philadelphia Union

Where: Audi Field.

When: Wednesday, 8 p.m.

TV: WJLA 24/7 News.

Records: United 7-10-6, 27 points; Union 11-11-3, 36 points.

D.C. probable starters: GK Bill Hamid; Ds Oniel Fisher, Kofi Opare, Steve Birnbaum, Joseph Mora; MFs Paul Arriola, Junior Moreno, Russell Canouse, Luciano Acosta, Zoltan Stieber; F Wayne Rooney.

Philadelphia probable starters: GK Andre Blake; Ds Keegan Rosenberry, Jack Elliott, Auston Trusty, Raymon Gaddis; MFs Haris Medunjanin, Borek Dockal, Alejandro Bedoya; Fs C.J. Sapong, Cory Burke, Fafa Picault.

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