"I thought if he took some anti-inflammatory [medicine] it would calm it down, but it's kind of been lingering," Johnson said of the injury. "When he runs, it swells up. He was jogging in the pool, it swelled up from that. We're concerned, so we'll get another opinion on it."
Harper was placed on the disabled list on Saturday, retroactive to May 27. He is eligible to be activated as early as Tuesday, but Johnson said that is unlikely.
The Nationals are 4-12 without Harper in the starting lineup.
"[We've] just got to get the swelling down, we got to get the inflammation out of there," Johnson said. "We haven't been real successful with our treatment here."
The reigning National League Rookie of the Year hurt his knee after running into the right-field wall at Dodger Stadium on May 13. He aggravated the injury on May 26 against the Phillies, when he made a pair of head-first slides and left the game early.
Harper said that he is trying to stay off the knee and rest it as much as possible. He said he would want to play in a few Minor League games and make sure the knee doesn't swell before returning to the Nationals' lineup.
"I'm not going to rush it. I'm going to take as much time as I can to get right," Harper said. "I want to get back in this lineup 100 percent. I don't want back in it at 80 percent."
Harper does not want to put any cortisone injections in the knee because he thinks that cortisone might damage it. Johnson said that there is not enough fluid in Harper's knee to be drained.
Doctors could remove the bursa sac in Harper's knee altogether, but that is usually the last resort. If that's the case, Johnson estimated that Harper could be out "probably two or three weeks."
"Hopefully [Dr. Andrews] is going to have some sort of better news for us," Johnson said.
Tom Schad is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.