Brady and Gronkowski have not publicly said they are committed to playing this year. Neither player participated in the team's voluntary workouts last week.
Brady, 40, threw for 4,577 yards with 32 touchdowns and just eight interceptions in winning his third MVP last season.
Gronkowski, 28, made 69 catches for 1,084 yards with eight touchdowns over 14 games in 2017. He is due to make $17 million over the next two seasons.
Gronkowski told NBC Sports Boston on Saturday that he is training for dirt bike racing.