It didn't matter, though, as Boston breezed to a 113-100 victory despite New Orleans getting 41 points and seven rebounds from Anthony Davis, who was cleared to start despite being listed as questionable with a right hip contusion.
Irving, who injured his right shoulder in the fourth quarter of Thursday's win over the New York Knicks, played 23 minutes in Saturday's evisceration of the Chicago Bulls, but Celtics coach Brad Stevens said Irving woke up Monday morning with shoulder stiffness, and Boston chose to be cautious with its star guard.
"Kyrie, I think, is going to be fine long-term," Stevens said. "He felt a little bit of stiffness today. ... I would expect that he'll be day-to-day."
Hayward, meanwhile, is out with an illness -- not anything to do with his recovery from the gruesome leg and ankle injuries he suffered six minutes into last year's season opener that caused him to miss the remainder of the 2017-18 campaign.
Perhaps most concerning is the absence of Horford, who has missed four of Boston's past eight games due to an ongoing bout with patellar tendinitis in his left knee.
Horford previously sat out one game and then came back before sitting out Saturday's win in Chicago and now Monday's game. Stevens indicated that this stint on the sideline might be longer to allow Horford to get completely over the tendinitis.
"Al has patellar tendinitis that he's just been dealing with for a while," Stevens said. "And so we're going to see how this goes. He's day-to-day right now, but we may go slowly with him."
Horford said before the game that it was difficult for him to sit out, but hopefully sitting out for a couple of games will prevent his knee issue from being a recurring problem.
"I don't want to have to keep dealing with this as a question mark game after game," he said. "I just have to listen to the training staff and what the doctors are telling me and buy into that plan.
"I went out there and moved a little bit. Still not where I need to be -- right away I could tell. But it is much better than two days ago. Hopefully a couple more days, and we can put it behind me."
In addition to Boston's three stars sitting out, the Celtics were without center Aron Baynes and forward Guerschon Yabusele against the Pelicans, as both are dealing with ankle sprains. Baynes, like Irving, suffered his injury against the Knicks, but unlike Irving, he didn't play against Chicago. Stevens said Baynes had gotten better but not good enough to play.
Yabusele, meanwhile, suffered a nasty-looking right ankle sprain in Chicago, and Stevens said it will be weeks before the second-year forward will be ready to return.
"Not even close to playing," Stevens said. "[It's] not a high ankle sprain, which is good news. The window is going to be at least a few weeks."
With all the absences, the Celtics started Terry Rozier, Marcus Smart, Jayson Tatum, Marcus Morris and Daniel Theis against the Pelicans and activated PJ Dozier, who is on a two-way contract, to give the team added depth.
"I think we already know a lot about the guys, some of the guys that are going to have to step up for us -- they've done that before," Stevens said. "But it'll be good to learn about some of the guys that haven't been in this situation.
"It's a great opportunity for all those guys. That's what the NBA [is]: You go to bed one night, and you're not in the rotation. The next morning you wake up, you better be ready to play."
While Davis started for the Pelicans, guard E'Twaun Moore was ruled out beforehand. Moore has been playing through a calf issue, and New Orleans chose to give him a night off to rest it. Guard Elfrid Payton also remains out with a fractured finger.