Hurt 1. Defensive Liability
First things first, Seth Curry isn't a bad defender. He has good anticipation, stays in front of his man the best he can, and holds a career average of 0.7 steals per game.
However, Curry is not a good defender. He has never had a positive defensive plus/minus, never provided more than two defensive win shares in a season, and has only had a steal percentage of more than 2.0 once in his career.
The advanced stats don't love Curry, and the eye test isn't overly kind as he doesn't have a large frame, nor is he a great athlete.
Curry is listed at 6-foot-2 and 185 pounds. Thus, Curry will be tasked with defending point guards or smaller two guards. Defending will be finicky for the Dallas lineup whenever Curry suits up next to Irving or Hardy. More on this later.
A trio of Doncic, Irving, and Curry will light up the scoreboard but will sieve points. To get the most efficient use of Curry, head coach Jason Kidd will likely pair him with a bigger guard or use him primarily against the opposing team's second-stringers.
The Mavs already fielded a bottom-5 defensive unit a season ago. This year, Jason Kidd must revive the Dallas defense in order to maximize the Mavs' potential. In the 2021-22 season, the Mavs reached the Western Conference Finals, and their defense coincidentally ranked in the top 6 in defensive rating.
Curry is a fantastic shooter, with gravitational pull on offense. But his defense isn't a strong suit, and the Mavs will likely be tasked with masking his deficiencies on the less glamorous end.