Westhoff sustained the injury, which is the same as the one troubling Fremantle ruckman Aaron Sandilands, when he stubbed his toe during the Power's last win against Richmond in Darwin in round 10.
The 24-year-old continued to play with the aid of pain-killing injections, but was unable to walk following the club's loss to the Brisbane Lions on Sunday and decided it was time to allow the injury to heal.
"It [the injury] has been lingering for a while and it's probably got to the stage where it's not getting any better," Westhoff said on Tuesday.
"It's not so much [bothering me] during the game because you get the jabs before the game and at half-time.
"It's more [an issue] during the week … about two days after the game I can't really walk. I've gone into wearing the moon boot for two days after the game to try and get it to settle down, but it's probably got the better of me over the last few weeks.
"I'm not able to train, which affects your fitness levels. It means your not getting out there and getting that physical conditioning, or going up against opponents during the week and that's probably affected my performance over the last couple of weeks."
Westhoff was optimistic he would miss only one game against St Kilda at AAMI stadium this week, but conceded the injury could keep him out until the Showdown clash with Adelaide in round 19.
The lanky goalkicker's absence could create an opportunity for veteran Chad Cornes, who hasn't played since round six.
Earlier this week, the club revealed Carlile would miss five weeks with a nagging hip flexor injury.
Small forward Simon Phillips dislocated his shoulder in the loss to the Lions, but remains a chance to take on the Saints on Sunday.
Katrina Gill covers Port Adelaide news for afl.com.au. Follow her on Twitter: @AFL_KatrinaGill
