D.C. United Coach Ben Olsen gave Paul Arriola high marks for his in-case-of-emergency performance at right back during a 2-1 comeback victory over the Chicago Fire on Sunday.

Arriola, a natural midfielder who dropped into the back line in place of Oniel Fisher, helped set up Wayne Rooney’s equalizer with a well-timed, well-placed cross. He was predictably active in the attack and made several one-on-one defensive plays in and around the penalty area.

With Fisher out for the year with a serious knee injury, Arriola would seem on track to retain the role Saturday against FC Dallas at Audi Field. But with Nick DeLeon back in the mix after a 4 1/2-month absence, Olsen might make adjustments in the next few matches.

“I thought he was great,” Olsen said of Arriola. “He gives us something on the offensive end and made some plays in our box that were extremely high level, defensively. His lungs are fine for that position. He is up and down and covers a ton of ground.”

But Arriola has also proved over the course of a long season that he is vital in a dangerous attack. His interaction with Rooney, Luciano Acosta and Yamil Asad, as well as his work rate and energy, have helped put United on course for an MLS playoff berth. Three victories in the last four matches would secure passage.

United (11-11-8, 41 points) is unbeaten in six straight and, in the race for the last Eastern Conference playoff spot, sits two points behind Montreal, which has played two additional matches.

Reading between the lines, it seems Olsen will insert DeLeon into the lineup as soon as the seventh-year pro improves match fitness. DeLeon was the incumbent right back when the season started, but a knee injury in late May sidelined him all summer.

On Sunday, he returned to the game-day roster and, with United clinging to the lead, entered in the 87th minute.

“We want to continue to see how Nicky responds, but so far, so good with him,” Olsen said. “It was nice to get him some minutes, just to get him out there.”

With a smile, Olsen added, “He was thankful for those minutes — too thankful. He was running all over the place, not doing what I told him to do.”

DeLeon’s maximum minute count at the moment is 60, Olsen said. In which case, DeLeon could start, catch his breath at halftime and leave the match “not long after that.”

>>> Choosing a right back to face Dallas, the Western Conference leader, isn’t the only decision awaiting Olsen.

International call-ups have left him without Venezuela’s Junior Moreno, a starting defensive midfielder; Costa Rica’s Ulises Segura, a part-time midfield starter who was in the lineup against Chicago; and reserve forward Darren Mattocks, a Jamaican who, despite a smaller role since Rooney’s arrival, continues to lead the team in goals with 10.

Chris Durkin — who, at age 18, has enjoyed a breakout season with 14 starts — is the clear choice to replace Moreno and partner with Russell Canouse. Zoltan Stieber (19 starts) would probably step in for Segura, unless Arriola returns to the midfield.

The loss of Mattocks leaves United without a natural backup to Rooney. The other forward, Dane Kelly, was also summoned by Jamaica (although he won’t report to camp for another day or two).

Moreno and Segura are also expected to miss United’s match next Wednesday against Toronto FC, but both Mattocks and Kelly will return in time.

Dallas will be without two starters (U.S. defender Reggie Cannon and Ecuadoran midfielder Carlos Gruezo) and one reserve (Honduran defender Maynor Figueroa).

Toronto, which has been eliminated from playoff contention, had seven players called up, including U.S. midfielder Michael Bradley and Italian forward Sebastian Giovinco. However, some are likely to rejoin the squad in Washington.

>>> On the D.C. injury front, center back Kofi Opare, who has missed three matches with a hip injury after starting 10 in a row, was back in full training Tuesday. Olsen said he would consider him for not only the game-day roster, but a starting slot.

“If he responds from [Tuesday’s rigorous workout] in the right way,” Olsen said, “he’ll be in the conversation.”

Elsewhere, goalkeeper David Ousted (elbow surgery) and defender Vytas Andriuskevicius (hip injury) have been involved in limited workouts but aren’t ready for match consideration.

>>> D.C. and Dallas are the two hottest teams in the league: United is on a 4-0-2 tear, while the Texas side has gone five in a row without a defeat (3-0-2) and leads the Western Conference by four points. Next on the surge list: The Philadelphia Union and New York Red Bulls are unbeaten in four straight.

>>> United has scored 29 goals in 12 appearances at Audi Field and 32 over 14 home games (the other two were at Maryland SoccerPlex and Navy-Marine Corps Memorial Stadium). At 2.29 per home date, D.C. is second in MLS behind Atlanta (2.5).

United’s 10-2-2 home record is third best in the league, behind the Red Bulls (13-2-1) and New York City FC (11-1-4).

D.C. will play the next three at Audi Field before closing the regular season at Chicago.