Quentin has been suffering from plantar fasciitis in his left foot.
Asked whether he expects Quentin back by the All-Star break, Guillen said, "No. I want to be optimistic more than anybody, but the process and progress right now, you have to be honest with yourself.
"If Carlos is back by the All-Star break, it's a [blessing]. He'd be lucky."
Guillen said that if the Sox aren't chasing a Central Division title in the second half of the season, Quentin could be shut down.
"If we are out of the pennant race, I don't see why he's got to come back and play," Guillen said.
White Sox general manager Ken Williams didn't sound as dire.
"I don't know where Ozzie gets his timetable from," Williams said. "Because we don't have a timetable for him."
Quentin said he jogged Tuesday for the first time in a while.
"It definitely was a positive, considering three days ago, I couldn't even start to jog," Quentin said. "Looking at it that way, it's over 100 percent improvement. Right?
"Patience is obviously a virtue. It's something I've never been good at, but right now it's something I'm forced to be. [I will] focus more on improvements I made. I'm more optimistic about those. We're getting good responses every day. I wake up and I'm not having setbacks, so that's good."
As far as returning by the All-Star break, Quentin said, "I don't want to make any promises, but I would definitely say that's one of the goals I would strive for."