Johnson, who's been platooning in center field with Kosuke Fukudome, could be out for at a month after fracturing his left foot Wednesday.
Johnson fouled a ball of his foot during a six-run first inning, then stayed in the game through the third before the pain was too much for him to continue. He was put on the 15-day disabled list Thursday.
"Usually when you foul a ball off your shin or foot, it takes a couple of innings and it starts getting better. And it kept getting worse. I couldn't put tons of pressure on it, so we figured we'd get it checked out," Johnson said Thursday.
X-rays revealed the fracture, the latest in a month full of bizarre injuries.
Pitcher Ryan Dempster just returned to the rotation after breaking his toe while hopping over the dugout railing. Catcher Geovany Soto is still on the DL with an oblique muscle strain he got from a batting practice swing.
"I can't remember what year it was in Toronto, but we had a similar year like that where it's just one thing after another and you couldn't believe all the things that were going wrong," Johnson said. "We're still right up there in the standings, which is what counts."
The Cubs began play Thursday one-half game behind St. Louis in the NL Central.
Johnson's stint on the DL is his second this season. He was sidelined earlier with back spasms. Chicago also played two months without third baseman Aramis Ramirez (shoulder) and now left-hander Ted Lilly is out with a sore shoulder.
Johnson said he expects to be back sooner than a month, noting that the Cubs have been able to overcome injuries with steady play from their bench and depth from their Triple-A Iowa farm club.
"We think Reed will be back in three weeks," Cubs manager Lou Piniella said.
General manager Jim Hendry said the Cubs hope to have Soto -- last year's NL rookie of the year -- healthy and back by the end of next week.
"You try not to dwell on it," he said of the injuries. "You can't wallow."