JUPITER, Fla. — Washington Nationals third baseman Anthony Rendon felt discomfort in his left knee while testing it out Tuesday, putting his status for opening day on April 6 in further doubt.

“It wasn’t a staggering pain but it was still there and still lingering,” Rendon said Wednesday. “There was no point in pushing it right now and possibly going back to square one. So we might as well fix it right now instead of later.”

Manager Matt Williams said Wednesday that a second MRI exam of Rendon’s knee taken Tuesday showed healing and no structural damage. It’s a matter of dealing with the lingering soreness in Rendon’s joint.

I think opening day’s in jeopardy,” Williams said. “At this point, it’s in jeopardy, because he hasn’t been able to do the baseball activity. That being said, he could come in tomorrow and feel much better, or at the end of this week and feel much better, and we can look at things. But right now, it could be in jeopardy because he simply hasn’t had the baseball activity to get ready. Does that mean he’s not ready for opening day but ready four days later? Potentially. We just don’t know at this point.”

Rendon tested out his left knee with rotational exercises, twisting and motions similar to swinging, on Tuesday. He sprained medial collateral ligament while making a diving play on March 9. “It’s not going as fast as everybody hoped, including Anthony,” Williams said. “It’s a tricky injury.”

Nothing is being done surgically or medically to Rendon’s knee other than rest, treatment and ice, Williams said. “It’s a sprain and it’s irritated and what we need to do is calm that down,” he said. “… We’re doing everything we can to treat it and get him back as quickly as possible.”

Originally, the Nationals referred to Rendon’s sprain as “mild” and said he would require a few days of rest. It has dragged on for more than two weeks. Rendon, who finished fifth in NL MVP voting last year, is an indispensable part of the Nationals’ lineup. The Nationals may have to replace him with a rotation of utility man Kevin Frandsen, second baseman-shortstop Danny Espinosa (who is, again, starting Wednesday at third base, a new position for him) or non-roster invitee Ian Stewart.

Rendon said he was scheduled to meet with Nationals trainers and a doctor on Wednesday to go over an evaluation and a treatment plan. He said he isn’t sure yet he if he can be ready for opening day. He and Williams both believe he has the type of swing that wouldn’t need much time to return to form but there is a dwindling number of games left with opening day just under two weeks away. Disabled list stints during the spring can be backdated as far back as 10 days before the season begins.

“I’m not usually that type of guy that needs a tremendous amount of time or at-bats to feel ready,” Rendon said. “But once I get my timing down, I’ll feel comfortable and I’ll be ready to go. I don’t think it’ll be a substantial amount of time.”

The Nationals could be without their three top hitters last season on opening day: Rendon, Denard Span (core surgery) and Jayson Werth (shoulder). Span, who could be out until May, has begun throwing and light swinging, and felt good. Werth is slated to take a “more aggressive” batting practice on Wednesday in addition to playing five innings of defense only in a minor league game in Viera, according to Williams. He could still be ready for opening day but his return remains a moving target based on his hitting progress.

In unrelated injury news, Nate McLouth’s surgically repaired shoulder is still bugging him when he throws. He can hit lightly and is running, but the throw remains an issue. McLouth’s status was already in serious doubt for opening day and even more so now.

“This is a long rehab,” Williams said. “He had extensive [labrum] damage in the there that had to be repaired. It’s bothering him to throw so they’re working through that strengthening process again. It can flare up from time to time.”