At this point, it's probably fair to write that Devin Booker's rookie season has been phenomenal. Once a rookie puts up four games of 30 points or more, he's the real deal.
Booker is averaging 12.3 points per game on 43/37/84 shooting splits, and virtually everyone admits that he has surpassed expectations for a 19-year-old.
But Booker is not the only 19-year-old of the past 15-20 years to start off his career strong. Though few players enter the league that young these days, a total of 25 guards since 1975 logged at least 300 minutes at the age of 19. There have been considerably more forwards and centers to achieve that feat, but let's stick to comparing Booker to other guards.
Of the 25 players, Booker's PER of 12.2 ranks 8th. His true shooting percentage ranks 2nd behind only Kyrie Irving. His PPG ranks 7th, and his win shares ranks 6th.
No one stat is perfect, but in general PER does a fairly good job of measuring offensive efficiency and prowess. I decided to compare Booker to some past 19-year-olds by taking the top 10 guards on that list of 25 in terms of PER and mapping out their progress over their first four seasons. The lowest PER in the top 10 was 11.6, and the highest was 21.4.
Two of the players in the top 10 are rookies (Booker and Russell), so for them there was nothing to map. But by looking at the other eight players who have had similar rookie seasons, we can get a good estimate of just how quickly these players develop throughout their rookie contracts. Those eight are Kobe Bryant, Kyrie Irving, Tony Parker, Jrue Holiday, Stephon Marbury, Ricky Davis, Martell Webster and Bradley Beal.
Below is a graph showing the average PER and average PPG for the eight players from ages 19-22.
Right now, Booker has a PER of 12.2 and is averaging 12.3 PPG. According to the graph, the average 19-year-old guard who had a similar season to Booker (in terms of PER) has a PER of 14.5 and scored 11.8 PPG.
Then, look at the following seasons. For a 19-year-old rookie, age 22 would be the final season of a rookie contract and the final season before he enters restricted free agency. A player's fourth season can often be the start of his "prime", with an absolute peak of the career usually coming at least a few years after that.
The average 22-year-old who had an age 19 season comparable to Booker's is averaging 19.3 PPG with a PER of 18.2
What does this mean for Booker? Well, if he remains the absolute average of all guards who had similar rookie seasons, it means he could be averaging about 19 PPG with an above-average PER by the time the Suns are preparing to offer him a contract extension.
Of course, it doesn't actually work like that. Every player is different, and there will be best-case scenarios as well as worst-case scenarios. But let's take a closer look at the pool of players Booker is being compared to.
Kobe Bryant is obviously the best-case scenario. Bryant, who averaged 15.4 PPG at 19, made steady progress until he was scoring 28.5 PPG by age 22 and dominating the league.
Kyrie Irving is the other obvious superstar who stands out, but Irving was surprisingly NBA ready even when he was 19. He has only increased his scoring by three PPG since his rookie year, and his PER of 21.5 this season is roughly the same as his age 19 PER of 21.4.
Though all three are point guards, Holiday, Marbury and Parker are good examples of players who had promising rookie seasons and then made consistent, steady progress for three years. By age 22, all three had entered their respective primes. Marbury was averaging 22.2 points and 8.4 assists per game for the Nets and would be named an All-Star for the first time just one year later. Parker averaged 17.5 points and 6.5 assists per game and would also be selected as an All-Star for the first time at age 23. Holiday made his only career All-Star appearance at age 22, averaging close to 18 points and 8 assists per game.
Bradley Beal is only in his fourth season right now, but he's a very interesting case to look at. The third overall pick of the 2012 draft had a very similar rookie season to the one Booker is having right now. He averaged 13.9 PPG, posted a PER of 13.6 and shot 41/39/79. However, his progress since then has been close to stagnant. Three years later Beal averages 17.7 PPG with a PER of 15.8 on 45/39/76 shooting. So while he is clearly at least a little better, he has yet to break out and play to his true potential.
Finally, you have two "busts" in Ricky Davis and Martell Webster. Davis actually averaged a career-high 21 PPG for the Cavs at the age of 23, but his NBA career didn't last into his thirties. He was instant offense off the bench for several teams but never carved out a role as a consistent starter who could help a team win games.
Webster, who was taken sixth overall in 2005, was waived in November of 2015. Webster is still only 29 but doesn't seem to have much, if anything at all, left in his tank. He was a sharpshooting role player for several seasons, but he never scored more than 11.4 PPG in a single season.
Despite having similar seasons as Booker in terms of PER, neither Webster nor Davis were nearly as successful at the age of 19. Davis played only 12 minutes per game and Webster averaged close to 18, but both are included in the data set because they still had relatively efficient seasons. However, just as Bryant's age 19 season is objectively much better than Booker's, Davis and Webster posted seasons that were objectively worse.
Additionally, injuries may have played a part in the hindered development of Webster and Davis. Davis played only seven games at the age of 21 due to ankle and knee injuries, and Webster played just five minutes in one game as a 22-year-old. Because I considered those seasons to consist of rehabilitation rather than pure development for each player, I included Davis' 1st, 2nd, 4th and 5th seasons in the data set, excluding the 3rd. For Webster I looked at the 1st, 2nd, 3rd and 5th seasons, excluding the 4th.
For Devin Booker, the future looks bright. There are outliers in progression to each side of him, such as Bryant on one side and Webster/Davis on the other. But the vast majority of players he is compared to made consistent, steady progress. Five of the eight players became All-Stars, and another one is still only 22 and looks like an above-average starting SG. As such, there are plenty of reasons to remain hopeful about Booker's development.