Comley, who has made 84 first-team appearances since joining the club from Dagenham & Redbridge in 2022, was forced off during the first half of Wednesday's pre-season defeat to Aston Villa.
The 28-year-old was taken straight to the hospital and underwent an x-ray to determine the extent of his injury with Sadler revealing that a fractured fibula will see Brandon face a spell on the sidelines.
Speaking via the Express & Star, Sadler said: "It was a fracture in his fibula from the game against Villa on Wednesday night so gutted about that one.
"Fortunately, it's not a load-bearing bone which then helps in terms of there's no surgery required.
"So you're talking six to eight weeks on that before he's back playing. At least three of those are taken up by pre-season games and that's the bit of positive news surrounding that.
"But Brandon will come back fitter and stronger and we look forward to when he does."