On his first full day back with D.C. United this week, Patrick Nyarko wanted to do everything on the practice field. It’s his nature.

But Nyarko was a no-contact player, his activity limited, and when he tried to rev his engines, the medical staff put a stop to it.

“I was a little angry,” he said Tuesday, “but I knew it was the right decision.”

The 30-year-old attacker, in his first season with United and ninth in MLS, is returning from his second concussion of the season and sixth of his career. He has not played in almost two months but passed league-mandated protocol and was cleared to resume training — gradually.

How does it feel after a light run? Do any symptoms return after a vigorous workout?

“I felt good,” he said after Tuesday’s session at the RFK Stadium training grounds. “It’s just grinding through with these guys again, my first full day, running around, getting a little fatigued. Game intensity is something different.”

By Thursday, Nyarko was involved in full workouts. He is likely to travel to Ohio for Saturday’s match against the Columbus Crew but, because he is lacking match fitness, he wouldn’t enter until the late stages.

United does need the help: In 18 regular season matches, the club has had eight scoreless performances and posted just 17 goals. In all competitions, D.C. has been shut out in 10 of 21 games and scored once in seven other outings. Before the latest injury, suffered May 20 at Philadelphia, the former Virginia Tech star was perhaps United’s best player: three goals, one assist and an influential presence both running at defenders and tracking back on defense.

With a history of head injuries, though, Nyarko and the staff have been careful about his return. If he feels even the slightest bit off, he pledged, he will withdraw from participation.

“There is a lot going on around the world,” he said of raised awareness and intensive research into sports-related concussions. “You’ve got to pay attention to it. The one thing I taught myself is to be smart and take the necessary precautions. I am doing all I can to control that part of it in terms of making sure there is nothing going on in my brain, in my head, and making sure I am fully healed before I come back. And I make sure the experts control that part.”

That means following advice from United’s doctors and medical trainers.

“They’ve done a good job trying to control me,” he said, “because honestly sometimes I can never control myself watching from the sideline. I want to be out there.”

Nyarko suffered his first concussion of the season in the March 6 opener at Los Angeles on head-to-head contact early in the match. He wasn’t hurt and remained in the game. By halftime, though, his condition worsened and he was replaced. He sat out the next match but cleared protocol and returned for the home opener a week later.

“I didn’t really think it was [a concussion], but they had to be extra cautious,” he said. “Obviously, I have to listen to the experts.”

The second one was serious. In the frantic final moments of the May 20 match against Philadelphia, he was defending in United’s penalty area when D.C. goalkeeper Travis Worra attempted to clear a high ball and launched his fist or elbow into the back of Nyarko’s head.

“I knew right away something was wrong,” he said. “The bus ride back was really bad, but the next couple days weren’t bad. So I thought, ‘Oh, maybe I am in the clear.’ And then all of a sudden, headaches, nausea, some confusion. Noise and lights affected me a little.”

United shut him down.

After weeks of monitoring and appointments, the symptoms subsided to the point where he was allowed to resume light workouts.

Coach Ben Olsen said he is following Nyarko’s lead: “It doesn’t concern me if it doesn’t concern him. These guys are big boys and they make decisions about their own careers. He feels good. He is excited to get back into it, and that is all I need to hear. … If the doctors say he is good to go, and he has passed his protocols, then it’s up to him.”

The long-term consequences of multiple concussions, and whether they lead to conditions such as chronic traumatic encephalopathy (CTE), are the subject of intense medical research and studies.

A National Academies Committee found that, “although studies of the effects of multiple concussions on cognitive function and symptom presentation have had mixed results, more studies report unfavorable changes than do not. The most commonly observed neurocognitive impairments have been in the areas of memory and processing speed. In some studies, symptom load (i.e., the number and severity of concussion symptoms) has been found to be increased in athletes with a history of two or more concussions.”

Nyarko’s second concussion occurred three weeks after Chris Rolfe, his longtime teammate in Chicago and now in D.C., suffered a head injury. Rolfe, United’s MVP last year, has not played since and isn’t expected back anytime soon. Veteran midfielder Davy Arnaud retired last winter because of a concussion.

Over the years, United has had a number of players sidelined by head injuries or forced to quit. Nyarko said specialists have told him he can continue playing, but ultimately he will have to decide when enough is enough.

“If there is something that comes up and I cannot go, it would be a tough position but I have to listen to it,” he said. “Until then, I have a mission to accomplish and I don’t want to let people down. The organization took a chance on me and I want to reward them. I have unfinished business. It will be hard if it’s cut short, but I will leave that to the experts and control what I can control.”