Johnson scored 30 points and the Hawks beat New Orleans 89-79 on Monday night, ending the Hornets' season-best seven-game winning streak.
Williams, who averages 14 points per game, was held out with an undisclosed lower back injury. He was examined by Hawks doctors Monday and will see another doctor Tuesday.
The Hawks, who had lost six of nine games, were told before the game by coach Mike Woodson they had to pull together to make up for the loss of Williams for an indefinite period.
"If he's out for the rest of the season, we can't worry about that," Johnson said. "We have to move forward with what we've got.
"It's a tough loss for us, but we've got to be strong and everybody has to step their play up," he said.
The rest of the season? Did Johnson know more about Williams' status than the Hawks were saying?
"I'm just saying, worst-case scenario," Johnson said.
Hawks general manager Rick Sund said it was too early to discuss Williams' status.
"Once we go through the whole process, we'll know more," Sund said.
Woodson called Williams' status "day-to-day" after the game.
The Hornets pulled no closer than five points in the fourth quarter. After a basket by Chris Paul cut Atlanta's lead to 82-75, Johnson set up a powerful alley-oop jam by Josh Smith with 2:22 remaining. Mike Bibby followed with a 3-pointer to push the lead to 10 points for the first time at 87-77.
"What we talked about doing to Joe we just didn't do," Hornets coach Byron Scott said.
Smith had 13 points and 13 rebounds for Atlanta, while Flip Murray had 14 points and Bibby added 10.
Johnson had only 12 points in back-to-back games and was held under 20 points in four straight before re-emerging against the Hornets. He had 10 points in the third quarter.
"We know Joe Johnson is the guy, we weren't able to put the clamps on him," West said.
Hawks center Al Horford said he could see a difference in Johnson early in the game.
"I don't know what he did or what happened, but he really picked it up for us tonight and really set the tone early," Horford said.
Johnson's big game helped Atlanta hand New Orleans its first loss since Feb. 21.
"It feels good, coming out in a playoff atmosphere against a good playoff team," Johnson said. "For us to respond the way we responded with Marvin going down and other guys stepping up, that's how we've got to play.
"If I've got to score 30 for us to win, then I'll come out and try my best to do it. But we have to do this collectively as a team. We're not doing to do it with one individual. We've all got to work together," he said.
New Orleans led 28-20 at the end of the opening period. The Hawks had eight players score in the second period as Atlanta outscored New Orleans 27-12 for a 47-40 halftime lead.
"The second quarter was awful, as simply put as that," Scott said. "I thought our guys in the second quarter did a terrible job of defending those guys. Whatever we could do wrong in the second quarter we basically did."
The Hornets' reserves scored only seven points.
"It's putting too much pressure on our starters," Scott said, adding he plans to make adjustments to his second unit.
"I can't sit there and continue to watch this," he said.
Johnson and Murray combined for four 3-pointers in the third period while Paul struggled with 1-for-5 shooting. Paul made 8-of-19 shots for the game.
New Orleans forward Peja Stojakovic missed his third straight game with back spasms. ... Johnson's 26 points through three periods matched his high total for the previous 11 games. ... Hawks guard Acie Law had three points and three assists while playing about 20 minutes.