With runners on first and second and two outs, Nationals third baseman Vinny Castilla doubled over the head of left fielder Gabe Gross to score Junior Spivey and Johnson, tying the score at 5.
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. He was listed as day-to-day for a week.
Johnson traveled to Chicago with the Nationals this weekend. In fact, on Sunday morning, Johnson played catch at Wrigley Field for the first time since suffering the injury, and he also was spotted riding the stationary bike in strenuous fashion.
However, he could not put any significant weight on the heel, and his plans to participate in batting practice were cancelled.
"It's hard to walk regularly without pain," Johnson said.
Johnson has been having a career year in 2005, hitting .320 with eight home runs and 42 RBIs.
"The most important thing is to get Nick healthy for the second half. He's a very important part of this offense and defense," Washington's interim general manager Jim Bowden said.
With Johnson out of action, the Nationals are expected to purchase the contract of outfielder Matt Cepicky, who was hitting .274 with 14 home runs and 58 RBIs for Triple-A New Orleans.
Cepicky played for manager Frank Robinson in parts of three seasons from 2002 to 2003 with the Expos.
"He gives Frank another left-hander off the bench. He led New Orleans in home runs and RBIs. I like his bat a lot," Bowden said.
Bill Ladson is a reporter for MLB.com.
This story was not subject to the approval of Major League Baseball or its clubs.