I still hold onto the belief that those 4 players can beat almost any other line up in the NBA. I believe that their games have a great synergy together and might only falter against the current Warriors, provided all are healthy. I do believe that this group can also be somewhat cost controlled over the next few years and be in constant contention. To help explain this, I will be looking at each player individually, starting with
Kyrie Irving: Say what you will about his leadership, but he is still the teams best player and we would not be as good without him. He is a great scorer, surprisingly good facilitator, and sometimes tries on defense. He will always be a score first guard, but he is still a lethal shooter at any range, has insane handles, and can get buckets at will. That is the player that can carry a team through the playoffs. He is also averaging a career high in assists and rebounds. I don't think Kyrie will ever fully buy into "equal opportunity" offense of Brad Stevens, that is just the type of player he is. However, I have noticed he does not go into full take-over mode unless the Celtics are struggling or it is a close game. For the majority of the game, he gets his points, but he also moves the ball really well.
Gordon Hayward: We are finally starting to see consistent flashes of "Old G", a dominant scorer is a great facilitator. His ability to (also) score wherever he wants is always valuable and he gets to the line a lot. His last three seasons in Utah he averaged 6 free throw attempts per game while shooting them at an efficient 82-85%. That is not a player you want to guard as the second option on offense. I find it helpful to think of him as the Klay Thompson to Steph Curry on the 16' Warriors team, except Hayward can actually put the ball on the floor. This leads to the other huge part of his game, his passing. Hayward ran point forward his last few years in Utah and was incredibly successful. This enables Kyrie to work off ball, allowing him to run around a ton of screens like Korver, and terrify the defense that way. Hayward's complete game is a constant threat to score and opens up Kyrie even more.
Jayson Tatum: I think he should the third option in this lineup and that is probably where he would thrive. He wouldn't have the defense constantly paying attention to him as they already have to contain Irving and Hayward. This means he is getting a weaker defender he can either blow past off the dribble, lose with cutting for open threes and layups, or take to the post. Tatum is a proven scorer and if he cuts more and gets to the line more, could be one of the most terrifying third options before even hitting his prime.
Al Horford: The glue that holds this team together. His is probably the second best passing big in the league after Jokic, a very good shooter, and a threat in the pick and roll. He can either pop or roll to the basket, where despite not posting up a lot, is still fantastic. His spacing and passing has been a huge part of the Celtics the past few seasons, and that becomes even more deadly when surrounded by the three players mentioned above.
The reason why I really like this grouping is that all four of these players are great shooters, three are great facilitators, they can all make their own shots in a pinch if they need to, and their style plays well off each other.
Just imagine having a Horford setting a screen for Hayward at the top of the arc as Irving is coming off multiple off-ball screens and Tatum is starting to set up in the post. All of this happens in about 4-5 seconds, what can the defense do? If you stick to Hayward as he starts to drive into the lane, he can dump it off to Horford for an open three or a cutting Kyrie. If you stay with Horford, Hayward has an open lane to the basket. If you lock down and paint and stay on Horford, Kyrie is almost surely open. If Kyrie is contained, that means that Tatum probably has a mismatch down low or in the mid post.
This was the version of the Celtics expected this year, but it can still happen next year and down the line. Kyrie and Hayward still have the max contracts, but Tatum is cost controlled for the five years and there is a decent likelihood that Horford stays on a smaller, long term contract. This team can and will be terrifying for the next few years if they stay together.