Duncan, 38, is in the final season of a three-year, $30 million contract. He is averaging 14.5 points, 10.0 rebounds, 3.1 assists and 1.9 blocks this season while shooting 48.5 percent from the field.
Popovich said Duncan will have a hard time walking away because of the success he has had this season.
"No matter how (the season) ends, I think Timmy is going to look at (retirement) again," Popovich told USA TODAY Sports on Tuesday. "And if you ask me, my guess is that he'll go for another one because he has been so consistent this season.
Last week, Duncan was named to his 15th All-Star Game and has been one of the few constants for the defending champion Spurs during an injury-plagued season.
"Manu (Ginobili) is not having a great year. Tony has had a really spotty year — he has been injured a lot," Popovich said. "Kawhi Leonard has been out most of the time, with his eye in the beginning and then with his hand. He is just basically starting his season. (Reserve point guard) Patty Mills came back in January after the shoulder (injury), and he's not back in it. (Center) Tiago Splitter is not having a great year so far. (Reserve guard Marco) Belinelli has been out, so it leaves one guy: Duncan. He is the guy who has carried us."
San Antonio is currently the No. 7 seed in the Western Conference with a 30-18 record. It has a three-game lead over the Phoenix Suns for the No. 8 seed and a four-game cushion between it and the No. 9 New Orleans Pelicans.
The Spurs host the Orlando Magic on Wednesday.
- Paul Palladino