Paul Arriola pursues the ball against Costa Rica's David Guzman during the U.S. national team's 2-0 victory last weekend. (Lachlan Cunningham/Getty Images)

D.C. United was undermanned in the late stages of Saturday’s preseason opener, so in need of a substitute after making 10 changes at halftime, Coach Ben Olsen borrowed someone from second-division Bethlehem Steel for the last 10 minutes.

Reinforcements arrived in Clearwater, Fla., a day later as Paul Arriola, Russell Canouse and Joseph Mora flew in from California after completing national team assignments — Arriola and Canouse for the United States and Mora for Costa Rica during a 2-0 U.S. victory in San Jose.

All three trained Monday and seem likely to play Thursday night when United will face MLS expansion club FC Cincinnati in Bradenton, Fla.

United did not invite many trialists to the first phase of Clearwater camp. Defenders Oniel Fisher and Chris Odoi-Atsem are rehabilitating and are not part of the delegation. Luciano Acosta was held out after returning the night before from France, where a potential transfer to Paris Saint-Germain went bust.

Wayne Rooney departed in the first half with a bruised right shin and Gabe Segal, an academy guest player, left with a calf ailment in the second half, prompting Olsen to request roster help near the end of the 1-0 victory.

United should have proper numbers Thursday with the three arrivals and Acosta likely in the mix.

Arriola is coming off a 90-minute U.S. performance in which he effectively stormed the flanks and scored a late goal. Mora entered in the 77th minute for the Ticos.

Canouse, however, did not play; in fact, he was not even in uniform. Six days earlier, he made the game-day roster but did not log any minutes.

The defensive midfielder, who was critical to United’s success last year, is not injured. It was a matter of Coach Gregg Berhalter deciding not to play him, which was odd, given there were two matches and up to six substitutions permitted in each game.

Berhalter, it seems, does not think Canouse’s passing ability is good enough for the new U.S. system that he is implementing, one in which possession is paramount. It also might be a case of Canouse failing to pick up the system as a whole.

In Berhalter’s debut against Panama on Jan. 27, veteran Michael Bradley started in the central role and Wil Trapp entered in the second half. In the Costa Rica match, Trapp was the starter and Bradley did not play.

The next U.S. camp is March 18-26 for friendlies against Ecuador in Orlando and Chile in Houston. Falling in an official FIFA match window, Berhalter will have all players available for selection. This past camp fell outside a FIFA window, preventing him from summoning players in-season in Europe and Mexico.

Arriola appears in prime position for another invitation; Canouse does not.

>> Rooney skipped both workouts Monday but was back at it Tuesday morning and appears on track to play Thursday at IMG Academy.

His injury, albeit minor, heightened the importance of acquiring a proven forward to provide cover and perhaps start in cases like this.

Olsen recently suggested testing midfielder Ulises Segura as a striker, but that is clearly not the long-term solution. Kyle Murphy, 26, was a late invite to camp, but with no first-division pro experience, he is a candidate more for Loudoun United, D.C.'s new second-division team, than for the first unit.

Last year’s back-up striker, Darren Mattocks, was not happy with his reduced role during United’s late-season charge. In the offseason, both sides agreed they should part ways. He is now with Cincinnati.

>> In the opener, despite scoring just once, Olsen was pleased with his team’s ability to build the attack out of the back -- a tactic he has emphasized at training camp.

“Some good offensive moments and the final stuff wasn’t there in the end,” he said. “But that’s been most of our focus the last week: building up and creating opportunities. There was some good stuff.”

>> Saturday’s match was originally scheduled to kick off at 4 p.m. Olsen asked to change it to 11 a.m. Why? In order to award the players with 1½ days off instead of just one. “They’ve worked hard," he said. “They earned it.”

>> After training Tuesday and Wednesday and playing Thursday, United will return to Washington for some time off and indoor workouts next Monday and Tuesday. Some roster decisions, and possibly additional trialists, are expected for the second phase in Clearwater, Feb. 13-23. In that stretch, United will play the second-tier Tampa Bay Rowdies and MLS’s Philadelphia Union and Montreal Impact.

>> Among the possible newcomers next week: D.C. academy player Griffin Yow, a U.S. under-17 national team attacker from Clifton, Va. He joined the team at the start of camp in the D.C. area but didn’t travel to Florida because of a minor injury. United has targeted the 16-year-old for a possible homegrown contract this year.

The other academy player in camp has been 16-year-old midfielder Bryang Kayo, from Montgomery Village, Md.

>> Thursday’s match will not be televised or live-streamed, leaving D.C. fans in the dark for the second consecutive match. In both cases, United is the visiting team. The first game came on the training grounds of Bethlehem’s MLS parent club, Philadelphia. The second will take place at Cincinnati’s preseason base.

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