On the play, Johnson bruised the heel while scoring standing up. He landed at home plate awkwardly while trying to avoid catcher Gregg Zaun, who was blocking the plate. Johnson was taken to Washington Hospital Center for an X-ray and an MRI. Johnson had to wear a protective boot on the way to the hospital.
Dr. Thomas said that Johnson had problems putting weight on the heel.
"When I saw it on TV, I thought he may have hurt his ACL," Thomas said. "It [looked] like he was thinking about sliding, changed his mind and tried to get around Zaun. Nick landed on a straight leg. That's why the entire blow landed on his heel. But his knee is good, and his ankle is good."
Johnson was not available for comment after the game, but manager Frank Robinson said that Johnson should have knocked down Zaun at the plate.
"[Johnson] was trying to avoid Zaun's leg. This is why catchers get knocked on their butts," Robinson said. "Zaun blocked the plate when he didn't have a play at the plate. As a runner, you should never avoid knocking them on their butts, because there is a good possibility you will get hurt."
Zaun, a former teammate of Johnson's when both were with the Expos, said that there was no intent to hurt Johnson at the plate.
"I think, from Frank's vantage point, he thought that I kicked Nick when he went by, but that wasn't the case. We're taught to hang in there as long as we can," Zaun said. "[Robinson] gestured at me, but I didn't take it personally. He should know me better than that. I'm not a dirty player. I'm hoping Nick's OK."
Injuries are nothing new to Johnson. He has spent time on the disabled list in five consecutive seasons because of assorted injuries.
Johnson is currently having the best season of his career. He is hitting .320 with eight home runs and 42 RBIs.
With Johnson out of action, Brad Wilkerson or Wil Cordero will start at first base.
Bill Ladson is a reporter for MLB.com. Writer David J. Selig contributed to this report. This story was not subject to the approval of Major League Baseball or its clubs.