The procedure was done Thursday in Dallas, the person tells The Associated Press on condition of anonymity because no official announcement was made. There is no timetable for Mills' return.
Despite his injury, Mills has agreed to re-sign on a multiyear deal with the Spurs, according to reports. Contracts can't be signed until July 10.
Mills emerged as a valuable backup to Tony Parker this past season and scored 17 points in the Spurs' Game 5 victory over the Miami Heat in the NBA Finals that gave San Antonio its fifth championship.