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On Jan. 11, the Boston Celtics defeated the Philadelphia 76ers in the NBA’s annual London showcase. It was a game nestled in between eight days of total rest — with two trans-Atlantic flights factored in — for the Celtics, who until that point had played the most games by far of any NBA team. With the No. 1 seed in hand, and more rest coming, the Celtics were in prime position to follow through on their surprising and thrilling hot start.

Fast-forward two months, and things aren’t so bad on the surface. Sure, the Celtics have slipped to second in the East, but they are a lock for the 2-seed, up six games in the loss column on Cleveland and up six on Indiana with a magic number for a top-two seed of just seven with 12 games remaining. The road to the Conference Finals will go through Boston.

But beneath the surface, there are a lot of reasons for concern.

WHAT’S WITH THE DROP-OFF?

Jayson Tatum predictably stopped leading the NBA in 3-point percentage. He’s still shooting a terrific 39% since Feb. 1 and has been particularly great as of late with Kyrie Irving out; he’s just no longer setting records. The Celtics’ defense remains No. 1 league-wide, a testament to how great it was the first three months of the season. It, also, is still good, as it ranks ninth league-wide since the London trip, but it’s trending down, 16th since Feb. 1.

Most notably, Irving has a negative on-court vs. off-court net rating on the season as a whole, which means the Celtics have outscored their opponents by more with Irving on the bench than with him on the court. Some of that is the result of how they performed in games he has missed with injury, such as their one-point loss to the Wizards on March 14. But some of it is that fact that the Celtics have a 109.3 defensive rating since that London trip with Irving on the floor, that’s compared to 100.4 with him on the bench.

A contributing factor?

The Marcus Morris-Kyrie Irving combo has a defensive rating of 114, which is wretchedly awful. That number drops by 11 points without Morris for Irving, and about the same for Morris. In other words, that specific combination doesn’t work.

But Irving is also really struggling just to keep guys in front of him. That may be due to his sore left knee, which has sidelined him the last few weeks. One thing that has been notable: the Rockets targeted Irving the way teams will target him in the playoffs. With P.J. Tucker at the 5, they spread Al Horford, the Celtics’ interior defender to the corner, and attacked Irving with … Trevor Ariza. That’s the guy who the Celtics tried to hide Irving on, and he still couldn’t keep up:

On top of that, Marcus Smart has suffered a torn ligament in his thumb that resulted in surgery and will force him to miss the rest of the regular season. There’s talk of him returning for the second round, but — spoiler alert — you can bet Smart will be available before then. For starters, shooting isn’t a concern for Smart; he’s terrible at it anyway. Secondly, John Wall played through a broken wrist in the playoffs, coming back in the same series. Smart is tough as nails — and will get a brace applied — and give it a go. He’ll help no matter what.

However, between Irving’s sore knee, Smart’s thumb, a season-ending injury for Daniel Theis, and the other bumps and bruises (not to mention the elephant in the room that is the Gordon Hayward injury), Boston is entering the postseason about as banged-up as it can get. Irving’s problems are obviously tantamount; no matter what his on-off numbers are, the Celtics will only go as far as Horford and Irving take them. But the bigger issue in the short term is that Boston succeeds because of its scrappy, the-whole-is-greater-than-the-sum approach, a trademark of Brad Stevens’ super-effort squads year after year.

Being short-handed hurts that approach. This Celtics squad is more talented than last season’s; Irving and Tatum are blue-chip players, and the whole roster is longer and more athletic. But it’s becoming more and more evident that Boston’s depth isn’t maybe as good as it was thought to be for half the season. And if the depth isn’t as good, and the star point guard is struggling … that’s not comforting.

THE ROAD

There’s still a month until the playoffs. If the Celtics can get healthy, they’ll be in a good position to advance, and that’s step one. But it’s important to note how expectations over the past six months have changed. We’ve gone from “the Celtics are a lock to make the Eastern Conference Finals” to “they should get out of the first round.” That doesn’t mean that they won’t make the conference finals; it means that the Celtics’ floor has shifted dramatically lower while their ceiling remains very much the same, which is a Finals appearance.

The nightmare scenario for Boston is Philadelphia in a 2-7 playoff matchup. A young, energetic team with no pressure on it, featuring a big man who can absolutely dominate Horford, and who brings shooting to the table, is pretty much a worst-case, first-round scenario. Horford is a Defensive Player of the Year candidate, but when he’s overmatched in size and athleticism, like he is against Cleveland’s Tristan Thompson, he struggles mightily. Joel Embiid is that on steroids. Embiid’s ability to deter Irving from his wild drives and incredible finishes makes it easier for Philadelphia to use its long perimeter defenders to stay home on Boston’s shooters, a crucial part of Boston’s pedestrian offensive attack.

But other teams present issues as well. Miami’s versatility and flexibility means that Erik Spoelstra can scheme 15 different ways to attack the Celtics, and Hassan Whiteside is neither as instinctive nor as gifted as other top centers in the league, but he’s a bear to deal with athletically. Throw in savvy from Dwyane Wade and Goran Dragic, and you have issues.

Indiana has versatility and balance. Despite having one of the best players in the league in Giannis Antetokounmpo, Milwaukee may honestly be the easiest first-round opponent. Washington is a group that can absolutely beat Boston and is always better in the playoffs than it is during the regular season. These are threats, even before we get to the second round.

Boston’s hoping for Indiana to keep pace, or for Cleveland to keep slipping, so that the Raptors have to deal with the Cavs in Round 2. The ideal scenario would be a Boston-Milwaukee first-round matchup, followed by a beatable Pacers team in the 2-3 matchup. Then the Cavs beat the Raptors, or the Raptors beat the Cavs; honestly, either way it works pretty well for Boston. Having to go through LeBron and then a Raptors team that finally looks legit, when it has home-court advantage?

That’s brutal.

On the other hand, the Celtics have more guys who can flip a series than most of those teams. Tatum’s poise and comfort mean that while he should struggle as a rookie, he might have a game where he just goes supernova, slips through the scouting report and flips a loss into a win. Boston’s model defensively is great, and the Celts have so many different guys on the bench who can go off. It’s not hard to look ahead and envision “The Terry Rozier Game,” or “The Marcus Morris Game,” where they just have a third quarter that disintegrates the structure of a team’s lead.

This season is mostly house money anyway for Boston. The Celtics lost their best or second-best player on the first night of the season and likely won’t get him back until next season. The Celtics are all but guaranteed to own the East at some point in the next three years, max. But after a start that helped them become the toast of the NBA and a squad that was simply superior, regression to the mean has started to take hold. Can Boston rediscover the magic of early November? Will their playoff gear be higher than their regular-season showing for the first time in the Brad Stevens era?

If they can, they’ll have momentum, one of the best young cores in the NBA, and get Hayward coming back after a Finals appearance. If not, there could be a few more questions we have to answer about this new Celtics core before we anoint them the NBA champs.

Top photo: Celtics guard Kyrie Irving; credit: Brad Rempel-USA TODAY Sports