The Boston Celtics enter the 2021 NBA preseason with one of the deepest training camp rosters they have fielded in recent years.

While the preseason remains as meaningless as ever for team projections, the closed training camp practices and four preliminary contests should help sift through an experienced group. Only one Celtic is a rookie (two-way player Sam Hauser), and 17 of the 20 players have at least two years of NBA experience. 

The competition should be fantastic throughout the depth chart. But some battles take on a higher importance as new head coach Ime Udoka finalizes his squad and shapes his team's identity. I have five groupings I'll be particularly interested in as opening night inches closer.

Robert Williams III vs. Al Horford — Who opens the season as the starting center?

This is the highest-profile "question mark" for the Celtics entering training camp. They just gave Williams a four-year, $54 million contract extension, and he finally started to string together healthy games in his third season. Horford, however, has more experience and notoriety given his last run in Boston. None of this is going to matter because Udoka will play whoever best complements Jayson Tatum and Jaylen Brown. 

Who had the NBA’s highest FG% inside the restricted area last season? Per NBA Stats (min. 100 attempts):

1. Giannis Antetokounmpo 💪 - 80.7%
2. DeAndre Jordan - 79.9%
3. Robert Williams III - 78.0%
4. Michael Porter Jr. - 77.5%
5. Daniel Gafford - 76.7% pic.twitter.com/QjL19obD94

— BasketballNews.com (@basketbllnews) September 12, 2021

Is that Williams, who was one of the most efficient interior finishers in the entire NBA last season? He's an eye-popping lob threat who can move the ball well for a paint presence and protect the rim at a high level. Horford, in contrast, can stretch the floor, pass the ball well and open up more versatile defensive approaches. He can theoretically play power forward as well, though I'm skeptical of that as a realistic option.

We'll see how Udoka deploys Tatum and Brown as playmakers. Perhaps the spacing with those two, Marcus Smart and Josh Richardson clicks, and Williams can finish plays as a roller or dunker spot threat. Or maybe the Celtics need a stretch big as they have in past seasons. Either way, both should receive heavy minutes regardless of their place in the starting lineup.

My prediction: Williams starts, barring injury.

Dennis Schröder vs. Payton Pritchard — Who runs the bench offense and when?

Which rookies led the NBA in “deep” threes made (>27 feet) last season? Per Sportradar:

1. Immanuel Quickley - 34 (38.6 FG%)
2. Payton Pritchard - 32 (40.5 FG%)
3. LaMelo Ball - 28 (34.1 FG%)
4. Saddiq Bey - 21 (35.6 FG%)
5. Anthony Edwards - 20 (27.4 FG%) pic.twitter.com/Ti8afYE0mS

— BasketballNews.com (@basketbllnews) September 9, 2021

Smart should be penciled in as the starting point guard and Richardson (more on him soon) appears to have the edge as a shooting guard. Schröder would then come off the bench as a starting-caliber point guard, while Payton Pritchard looks ready for a larger load after his dominant summer league.

What responsibilities would they each have? The 28-year-old Schröder is a seasoned lead guard who can attack off the dribble and finish at the rim. Pritchard doesn't have the same explosiveness, but instead has impressive range as a perimeter shotmaker. He orchestrated the Celtics offense in Las Vegas and became a backup point guard by default as a rookie. Is that enough to take responsibility off of Schröder — and how will each fare without the ball in his hands?

My prediction: Schröder gets more lead-guard run as a capable veteran.

Josh Richardson vs. Aaron Nesmith vs. Romeo Langford — Who capitalizes on available minutes?

Josh Richardson on getting vaccinated: "That subject is pretty personal. I talked about what I'm going to do ... with my family and my circle. I'm not really sure. I think it's good that people are still educating themselves on the subject. Going forward, I'm not sure."

— Souichi Terada (寺田惣一) (@SouichiTerada) September 27, 2021

Richardson should be starting for the Celtics as a two-way wing due for some positive regression. But the vaccine situation matters, from a public health and player availability perspective. Yes, he does not say he is unvaccinated outright, but this is a pretty loud hint that Richardson could miss opening night against the New York Knicks, and potentially more games. 

In the possible absence, is Aaron Nesmith ready to step up? The second-year wing flashed off-dribble attacking ability in summer league and is a known three-point threat. But Nesmith has an injury history, as does third-year guard Romeo Langford. The No. 15 pick from the 2019 draft has played just 50 games in his NBA career and is still a relative unknown. Who will we see in what roles, and when? 

My prediction: Absolutely no idea.

Grant Williams vs. Juan Hernangomez vs. Garrison Mathews — Who is the stretch 4 off the bench?

Boston brought aboard Hernangomez, Mathews and Hauser this summer, and each slots into that hyper-specialist, sharpshooting forward role. With context, Hernangomez would seem to have the edge as the primary piece of a trade from Memphis, while Mathews is currently on a non-guaranteed contract and Hauser is signed on the two-way deal. But they'll ultimately have to fit with the incumbent in Williams.

The Celtics' other 2019 first-round pick has been... interesting through his first two seasons. He is woefully streaky as a shooter, registering three months over 40% from three last season along with two under 32%. He's also a 6-foot-6 forward who works really hard on the defensive end, but can be outmatched due to his size.

It would be surprising to see Williams out of the rotation entirely, but Udoka could drastically alter his role depending on how he gets going this fall.

My prediction: Williams and Hernangomez stick in the rotation, but Hernangomez has more offensive responsibility.

Which veterans hang onto the end of the roster?

Jabari Parker, Theo Pinson, Ryan Arcidiacono, Juwan Morgan and Mathews are all signed to non-guaranteed training camp deals or partially-guaranteed contracts. The Celtics only have one active roster spot and one two-way contract available. Five veterans are essentially fighting for two spots, and that does not even include Bruno Fernando, who is on a fully-guaranteed deal as the fourth center on the depth chart.

That's a pretty serious battle for six veterans. The NBA G League website says that players in their fourth season or earlier are eligible for a two-way contract. This would make Fernando, Pinson, Morgan and Mathews eligible two-way players, though I would be suprised if a 24-game starter in Mathews were to opt to stay on a two-way deal. 

Do the Celtics want steady guard depth in Arcidiacono and his 5.6 assist-to-turnover ratio from last season? Or do they opt for a scoring forward in Parker or Mathews? What about keeping the guaranteed-contract player they have in Fernando? And what about that last two-way slot?

Ultimately, these will be the most pressing questions during preseason action since all of these players should get quality playing time.

My prediction: Fernando and Mathews get the final active roster spots. Pinson gets the two-way.

Boston may not have a title-contending roster on paper. But it's clear that Udoka and Brad Stevens are building for the present and searching for the best complements to Tatum and Brown. As the two young stars creep towards their primes, it's becoming time to open the Celtics' window of opportunity.