While Rivers said he would not be disciplined for the mix-up, he didn't sound particularly pleased about O'Neal's absence on a day the team practiced with 11 players.
O'Neal joined the team in time for its afternoon flight to Cleveland in advance of Tuesday's game against the Cavaliers.
"He got the times mixed up," said Rivers. "He missed one, so he owes us one. We're good."
Rivers even kept it light when asked about potential punishment.
"No, I was going to spank him, but it might not hurt," Rivers joked. "It may hurt me."
Rivers gave his team the weekend off after Friday's 110-101 triumph over the Toronto Raptors. Players were supposed to report back for a noon ET practice followed by a 3 p.m. flight to Cleveland. Every other Celtics player -- including injured ones like Kendrick Perkins, Jermaine O'Neal, and Delonte West -- were present for the session.
Rivers admitted the absence was frustrating, but he said he'd give Shaq the benefit of the doubt this time around.
"Yeah [it's frustrating], but you get over it," Rivers said. "Listen, if the guy had a history of something, then you'd deal with it differently. But when the guy doesn't have a history of something -- it happens. It's life, you move on.
"It's not a big deal."
Rivers pointed out that he doesn't have set rules for any individual player and lets their history and reputation dictate how they are handled.
"If you have a history of [missing practice], we deal with it," Rivers said. "It's a human game, it always will be. I don't have a set rule for one guys or two guys. Each guy is an individual basis, is what I'm saying. You deal with it that way. You put money in the bank and, if you're a guy that tends to use up your money, then [the leash] is a little shorter."
Pressed on the specifics of the incident, Rivers ended the Shaq chatter.
"I'm not getting into all that stuff," Rivers said. "It's done, it's over. We've moved on. I'd like you guys to move on."
The Celtics are dealing with a variety of injuries, which made Shaquille O'Neal's absence more of a concern. Rivers gave a rundown of the walking wounded.
West, who fractured his right wrist in Wednesday's win over the New Jersey Nets, is scheduled to have surgery Tuesday morning and the team expects to have a better idea of his return timetable following the procedure.
Jermaine O'Neal is rehabbing at the team's practice facility, but entering the third week of the initial two- to three-week timetable suggested for his return from lingering knee soreness, Rivers said the backup center still wasn't close to getting back on the floor.
"No, he's in town, he's working out, and doing all the stuff he should be doing," Rivers said. "But I haven't even checked [on his status], honestly. He's not playing this week, I can tell you that."
After missing three games due to a strained left hamstring, Rajon Rondo returned to the court Friday night against Toronto and participated fully in Monday's session. Both player and coach suggested the hamstring is no longer a concern, but the plantar fasciitis he's been battling is on the radar.
"He feels pretty good, but the foot is still the bigger issue," Rivers said. "The hamstring is fine, for the most part. But with [plantar fasciitis], you have to be diligent. We do give him rest. Today, we only had 11 guys, which sucks, it hurts your practice. It means most of your guys are on the practice floor the entire practice. That's not the way you want to go."
Rondo admitted the feet were a bigger concern than his hamstring, but downplayed any of the ailments
"I'm fine," said Rondo. "I'm playing, so I'm healthy."
Chris Forsberg covers the Celtics for ESPNBoston.com.