Coach Jacque Vaughn said before the Nets played Washington on Wednesday night that Simmons was feeling pain in the back of his knee when he asked to come out of their victory over Orlando on Monday night.
The injured turned out to be the calf, not the knee.
Simmons also will miss home games against Toronto on Friday and Boston on Sunday before he is reassessed.
Vaughn said Simmons didn't recall a specific moment when he was hurt and that it could have just been caused by playing in a number of games after missing last season. Simmons missed five games earlier in the season with left knee soreness.
The Nets also said Yuta Watanabe would remain out and be reassessed next week because of his strained right hamstring. The forward was leading the NBA in 3-point percentage when he was hurt after a victory over Memphis on Nov. 20.