The Boston Celtics' perfect record was finally blemished Monday night versus the Chicago Bulls. After a red-hot first quarter from the C's that saw them up 19 at one point, the Bulls ran wild and beat Boston 120-102.

The Green Team now sits at 3-1, which should be thought of as a success considering their tumultuous offseason. Yet, their road loss to the Bulls was a brutal one.

Boston had a double-digit lead in the first but blew it by half, and its defense looked subpar at best. Plus, head coach Joe Mazzulla was ejected along with forward Grant Williams, so the frustration from the game was clearly getting to the Celtics.

While it's no time to panic this early in the year, there are some red flags from this loss that the Celtics need to address. With that being said, here are the three biggest concerning trends from Boston's first loss of the 2022-23 NBA season.

3. The Celtics don't have the same defense without Robert Williams III

The season has just begun, but the C's clearly miss Robert Williams III. Their towering center is out for at least the next few months, and Boston desperately needed his shot-blocking abilities against Chicago.

So far, the Celtics haven't been able to hold any of their opponents under 100 points. That's not shocking in today's NBA, however, it is cause for concern when the Celtics are supposed to have a top-tier defense.

After largely containing some of their opponents' best players for the first three games of the season, Boston couldn't hold  down Chicago's stars. Zach LaVine, Nikola Vucevic, and DeMar DeRozan combined for 62 points, and the up-and-coming Ayo Dosunmu added 22 of his own.

These defensive problems weren't really turnover-related either, as the Celtics had 10 to Chicago's 13. At times, the issues appeared to stem from a lack in effort, size, and communication.

Although Timelord can't solve all the defensive issues the Celtics are having, there are few players in the league who can guard the paint like him. Chicago outscored Boston 46-34 in that precious area, and it was evident that the looming presence of the All-Defensive Second Teamer was dearly missed.

2. Boston lacks strong rebounding 

Defense isn't the only reason why Boston needs Timelord back. The Celtics' team rebounding was atrocious against the Bulls, as Chicago nabbed 15 more boards than them.

That disparity doesn't quite do the game justice, though. To put it in perspective, Vucevic had a whopping 23 rebounds in 31 minutes. He's a great rebounder, but Boston's bigs were simply getting embarrassed on the boards.

This wasn't only an issue against the Bulls, as the Celtics were also outrebounded 48-37 versus the Orlando Magic. Without the height of Timelord, the C's are giving opposing offenses second chances and allowing teams like Chicago to get back into games.

Boston's key big men, Al Horford, Grant Williams, and Noah Vonleh, combined for a measly 13 rebounds. For a team as physical as the Celtics, that's startling and will certainly lead to more problems down the road if they don't correct it.

1. Joe Mazzulla still has to prove himself

After a wild offseason that crescendoed with Ime Udoka's one-year suspension, Joe Mazzulla found himself as the new head coach of the Boston Celtics. He has no prior NBA head coaching experience and is a whole two years younger than Horford, yet he has led Boston to a solid 3-1 start.

Of course, it's hard to tell how much of that success is the coach's doing. Once the C's started to lose their composure against Chicago, he took a very long time to stop the bleeding with a timeout. Some took to Twitter to voice their concerns with his decisions:

While this blown lead isn't Mazzulla's sole responsibility, it does show that he hasn't fully proven himself as a head coach. He deserves a fair amount of slack given the Udoka situation, but Boston's failure to respond to adversity against the Bulls should be something to monitor. Hopefully, it isn't a recurring problem early on like last season.

After this rough loss, it's clear the Celtics have work to do on both sides of the ball. Yet, overall, it has been an impressive start to the season. The C's will get a chance to bounce back at home against Cleveland on Friday, which will be another difficult Eastern Conference battle. But these are the type of games Boston needs to put itself with the rest of the NBA elite.