Jefferson, the team's leading scorer and rebounder, has been hobbled since straining the plantar fascia in his left foot in the first quarter of Game 1.
He didn't participate in Monday's shootaround.
"He's not close to where he was for the last two games," Clifford said.
Clifford said Jefferson's foot got worse in Game 3 as the game progressed and that having only one day off between games didn't help his chances of being ready for Game 4. Jefferson hobbled noticeably Monday as he left the gym after watching the team's shootaround.
Jefferson said he feels as if he's "walking on needles."
"I wouldn't wish this pain on anyone," Jefferson said. "It's the worst pain I've ever dealt with in my career."
Jefferson wouldn't rule himself out, saying he's taking things hour-to-hour.
"I'm seeing how it's going to feel and go from there," he said.
Miami leads the best-of-seven first-round series 3-0.
Clifford said Bismack Biyombo is expected to start for Jefferson.
That represents a major drop-off in production, particularly on the offensive end. Jefferson averaged 21.8 points and 10.8 rebounds per game in his first season with the Bobcats, while Biyombo has never scored more than 14 points in a game.
Biyombo has played only 15 minutes in the playoffs and scored one point.
Jefferson's teammates said they respect his ability to get on the floor the past two games.
"He has given us all he can," point guard Kemba Walker said. "And I think down to a man we appreciate his effort. He's been nothing but great for us. He has been playing really well and he's not 100 percent."
Said Clifford: "I know how badly Al wants to play. The most upset person about this is him. So it's frustrating, but again it's part of pro sports."
Clifford said the Bobcats will play more five-man basketball using pick-and-rolls rather than relying on Jefferson in the low post. Walker, the team's second-leading scorer, will be relied on to help pick up the slack.
"We have to find better ways for Kemba to be on the attack," Clifford said.
Jefferson said it's frustrating that he worked so hard all season only to injure the foot in the first quarter of the playoffs.
"This season has been a great season for all of the things we have done so far," Jefferson said. "We are going into the game tonight trying to win. ... Regardless of what may happen, it has been a wonderful season and guys have a lot to be proud of for the things we have done this season. But we can't take any steps back. We have to continue to build and get better."
Jefferson said he won't need surgery after the season.