I love Marcus Smart and felt the pain of losing him in the blockbuster trade for Kristap Porzingis. Initially it was tinged with disappointment over losing our "heart and soul" leader of this team. That is unquantifiable, and will be a default argument as to why trading Smart rather than Brogdon makes the Celtics worse (assuming KP comes here in either case). Recency bias also factors in here: Brogdon became progressively awful in Heat series due to the injury to his elbow, and this obscures how well he played for us during the entire regular season and much of the playoffs.
Objectively, however, I can't help but conclude that Brogdon played significantly better for the Celtics than Smart last regular season. Looking at their stats, it is hard to argue otherwise. Per 100 possessions, Brogdon was much more efficient in FG% (48.4% v 41.5%), 3P% (44.4% v 33.6%, on same volume! (8.3 v 8.5 attempts)), FT% (87% v 74.6%).
This resulting in a net positive rating for Brogdon of plus 9 and minus 1 for Smart (Smart's defensive rating was only 1 point better, 112 v 113).
This exceptional difference in efficiency resulting in Brogdon outscoring Smart by over 10 points per 100 possessions (28 v 17.5).
Brogdon was a significantly better rebounder (7.8 v 4.

. Smart also had nearly one more turnover per 100 than Brogdon (3.6 v 2.7).
To be fair, Smart averaged 2.6 assists more per 100 (9.5 v 6.9) and averaged 1 more steal (2.3 v 1.3).
Their blocks were near identical per 100 (0.6 Smart, 0.5 Brogdon).
The advanced metrics build up an even stronger case for Brogdon: Brogdon had an OBPM of positive 2.3 (2nd best on the team behind Tatum (4.

) and Smart's OBPM was minus 1.8, ranking 12th on the team. Smart's DBPM was only slightly better than Brogdon's (0.8 v 0.6), leaving Brogdon with the 4th best BPM on the team (plus 2.

, and Smart with the 10th best (minus 1). Brogdon trailed only Tatum, Rob and Al in BPM, and was better than Derrick White (plus 2.4).
Brogdon also had the 4th best Value Over Replacement Player (VORP) at 2.1, trailing only Tatum, White and Horford, while Smart was 9th place at 0.5 VORP.
I suppose some of this stark differences could be attributed to Smart playing starter minutes vs Brogdon coming off the bench. I would not know how to factor that in statistically. Open to suggestions.
This is not an attempt to bash Marcus Smart. Love the guy, and wish him the best. Just want to point out that Brogdon before the injury late in playoffs had an impressive season, which of course was acknowledged by winning 6th man of the year. The John Havlicek Award.
I hope we keep him.
Below are highlights of some of his best plays last season.
https://www.youtube.com/watch?v=OYi4GoUkDIs