Mirotic averaged 10.8 points over his first three seasons with Chicago. A restricted free agent, he signed a two-year contract that could pay as much as $27 million in September. The club holds an option on the second season.
A 2015 first-round pick, Portis has averaged 6.9 points and 5 rebounds.
The Bulls open at Toronto on Thursday. Chicago is rebuilding after trading Jimmy Butler and parting with Dwyane Wade and Rajon Rondo.