"I'm somewhat concerned about Sunday," Yankees manager Joe Girardi said. "He hit himself hard."
Johnson, 31, was working with a 2-2 count against Baltimore starter Alfredo Simon when he connected for the foul ball, which ricocheted off the inside of his right knee. He was helped off the field by assistant athletic trainer Steve Donohue.
The Yankees diagnosed the injury as a bone bruise, classifying their designated hitter as day-to-day. Hours later, after extensive ice treatment, Johnson was sporting a heavy bandage on his knee, blaming himself for chasing the low-and-inside pitch.
"I should have swung at a strike," he said. "It hurt pretty bad."
Asked to state the odds he would be ready for Opening Day, Johnson said simply: "Yes." But Girardi, who is currently fretting over minor injuries to Jorge Posada, Francisco Cervelli, Damaso Marte and Alfredo Aceves, was not quite so optimistic.
"It's frustrating, because we were extremely healthy," the manager said. "You kind of wish you had another week to make sure these guys are good."
No tests were performed on Johnson, who finished the Grapefruit League season with a .256 (10-for-39) average, three home runs and eight RBIs.
Having signed Johnson to a one-year, $5.75 million contract to help replace departing DH Hideki Matsui, the Yankees have been concerned with Johnson's health this spring. Johnson owns one of the game's more extensive injury histories, having played an average of 93 games per season since becoming a full-time starter in 2002. His history is also one of fluky injuries, including a broken leg suffered in a collision with then-teammate Austin Kearns that forced him to miss the entire 2007 season.
Johnson was sidelined early in camp this spring by a stiff back, which he suffered after catching a shoe spike in a turf mat around the batting cage.
Girardi said he has not yet created a contingency plan if Johnson cannot play in Sunday night's 8:05 ET opener against the Red Sox. One option would be to start outfielder Randy Winn at designated hitter, while sliding Curtis Granderson up to second in the batting order.
But the Yankees first must wait for an update on Johnson.
Barring a miraculous recovery, Johnson will not play in the team's exhibition against the Yankees' Future Stars on Saturday, a game that will be shown on MLB.TV at 1:05 p.m. Instead, he will rest and receive more treatment, in hopes that he will be well enough to play on Sunday.
Johnson is 3-for-9 with a triple and four walks in 13 career plate appearances against Sunday's Red Sox starter, Josh Beckett.
Anthony DiComo is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.