Eddie Johnson, left, with Davy Arnaud last season. (By Tony Quinn)

The last time D.C. United visited Red Bull Arena, Eddie Johnson was in the starting lineup for last fall’s Eastern Conference playoff match. This Sunday, United will return to Harrison, N.J., for the first away test of the 2015 MLS campaign. The veteran forward will not start. He will not be in uniform or inside the venue.

Out indefinitely with an unspecified medical issue, Johnson might not play again for United or, for that matter, any club.

While United officials have remained mum and Johnson has not spoken about his condition — one that sidelined him for the entirety of the six-week training camp — multiple sources say the ailment involves his heart and threatens to derail his career.

“Maybe he comes back, but it’s a long shot,” said one source. “No one knows for sure yet.”

Johnson, who turns 31 on March 31, has seen countless specialists around the country. Club officials hope to know more about his status this month.

Speaking in general about Johnson’s absence, Coach Ben Olsen said: “It’s a terrible situation for Eddie, but I’ve spoken with Eddie, and Eddie gets it. There is a potential life issue here that has to be focused on and making sure he is not putting himself in any danger to play this sport again. That is all that matters.

“It’s disappointing, but he has had a good career. I stopped when I was around his age. He played at a higher level than I did. He started pro when he was 16-17. That is a heck of a career. He should appreciate those years. I know he is thinking about the next stage in his life, if it comes to that.”

Johnson, a U.S. national team alum in his 15th pro season, joined United from Seattle before the 2014 season and became the club’s s highest-paid player. In 2014, his total compensation was about $613,000. He scored seven league goals, third most on the club, but did not meet expectations and was relegated to a secondary role late in the season.

Johnson’s heart problem first came to light at the conclusion of the season. On a late Thursday night, less than two days before the second leg of the Eastern Conference semifinals against the New York Red Bulls at RFK Stadium, he was not feeling well and checked into a hospital. Doctors told him he was suffering from dehydration and high blood pressure. He was kept overnight and missed practice the next day. Although cleared to play, he was not ready to start in the finale. He did enter in the 60th minute.

A few days later, Johnson went into detail about the medical episode and said he had an enlarged heart, which is not uncommon among athletes but can mask deeper problems.

“It was really scary,” he said at the time. “They did an echo, CAT scan on my brain and chest. Everything was fine.”

Johnson reported to the club in late January, but on the eve of camp, United announced he would undergo further testing and miss the three-week Florida phase of training. No other information was released. His agent, Richard Motzkin, also declined comment, saying he was respecting Johnson’s privacy.

The absence extended through preseason in Texas, the two-leg CONCACAF Champions League quarterfinals and the MLS opener against the Montreal Impact on March 7.

“Eddie wants to play,” another source said. “Right now, he has to do what’s right for his health.”

For United, the cold reality is that a potential starter on a high salary is not available for the foreseeable future.

Last year, as a designated player, Johnson counted $387,500 toward the $3.1 million salary cap, or 12.5 percent. (The balance of his earnings were on United’s books instead of MLS’s.) The league has yet to outline this year’s salary and roster guidelines, but any DP contract would take a considerable chunk of the salary cap.

Johnson’s contract is guaranteed, so if United chose to release him, it would gain a roster slot but his contract number would continue to apply toward the cap.

MLS does permit clubs to buy out one guaranteed contract per year for the purpose of clearing cap space, but only during the offseason.

From MLS’s 2014 Roster Rules and Regulations:

A Team may buy out one (1) guaranteed player (including a DP’s) contract during the off-season and free up the corresponding budget space. Such a buyout is at the particular MLS Team’s own expense.

A Team may not free up budget space with a buyout of a player’s salary budget charge during the season. Such a buyout will be conducted by the League and count on a Club’s budget in a manner consistent with current MLS guidelines.

Whether MLS would make an exception for an extreme hardship such as Johnson’s is unclear. Cap space would come in handy this summer, when the international signing period re-opens.

There are other issues at work here, as well.

Was United aware of a pre-existing condition before acquiring him? If Seattle knew of the problem, did the Sounders have an obligation to inform United? Assuming Johnson knew of his condition, did he say anything to United before agreeing to a sizable contract? Did Fulham, which owned his contract from 2008 to ’11, know anything?

Was MLS aware before signing him in early 2012 and assigning him to a club through the allocation order? If so, did the league inform the teams?

Or was Johnson’s condition known to all and simply worsened over time to the point where he had to stop playing?

If Johnson is forced to retire, what would happen to his guaranteed contract and its impact on United’s salary cap? Would the league and team continue paying him? And would D.C. recuperate cap space?

The saga is likely to drag on for months, two sources said.

Until matters are settled, Olsen will conduct business as usual with whom is available.

“I like this group. We’re capable of winning games with who we have,” Olsen said. “And I am comfortable with our depth once [Markus] Halsti, [Luis] Silva and some of these [injured] guys getting into the fold. Then we’ll evaluate who we are. I’m sure something will spell itself out where we might need this position or that position, but for now, it’s a good group and I’m looking forward to see how we do over the first quarter of the year, and then we can adjust after that.”

United notes: Silva (hamstring) and Halsti (knee) have not returned to full workouts and seem unlikely to travel to New York. Left back Taylor Kemp (groin) is back in the mix. … Midfielder Ian Christianson (Georgetown) was not offered a contract after completing a two-week trial. … Alan Kelly is Sunday’s referee. … United will take Amtrak to the New York area Saturday. … ESPN2 will carry the match, with coverage beginning at 5 p.m. ET.