Regret #2: Not seeking to trade Patty Mills and Rudy Gay
Former Spurs Patty Mills and Rudy Gay found new homes this offseason, after signing with the Brooklyn Nets and Utah Jazz, respectively. Although their departures were expected, the Spurs' decision to keep them past the trade deadline was questionable. Perhaps the Spurs expected to have the option to sign-and-trade them in free agency, much like with DeRozan, though they ended up with nothing.
This was not the first instance of the Spurs' questionable asset management, as many Spurs fans had similar concerns after the buying out of Lamarcus Aldridge's contract. To the Spurs' credit in this instance, though, the decline that Aldridge experienced appeared to be very sudden, and they may not have been able to find a worthy trade candidate before teams caught onto his decline.
With that in mind, though, both Gay and Mills seemingly have the stamina to play in a useful role for several more years. They were reportedly drawing interest from the Miami Heat and Los Angeles Lakers, respectively, and the Spurs may have been able to accumulate more draft assets had they shipped them off before the trade deadline.
With San Antonio in rebuilding mode, letting two assets go without receiving anything in return is hard to justify and something another team in their position likely wouldn’t have done. Thus the decision not to trade Mills and Gay before the start of the offseason or work out a sign-and-trade is certainly an offseason regret.