Scans cleared the 22-year-old of structural damage after he hyperextended his left knee in Sunday's pre-season win over Sydney at Etihad Stadium.
Stanley suffered the injury in a marking contest and he was subbed out of the game immediately.
Coach Scott Watters admitted post-match the Saints initially thought Stanley might have suffered serious damage to the joint.
"It was discovered that Stanley had suffered no structural damage, but some bone bruising," the club said in a statement.
They added he would miss between two to four weeks, meaning he is unlikely to play in their round-one match on March 30 against Gold Coast.
Stanley could become a key to his team's fortunes this season, with the Saints successfully trialling him as a key defender in Sunday's win.
He kept Sam Reid goalless and impressed Watters with his performance.
The Saints need to bolster their defence and Watters said Stanley had shown potential in the role.
"He's obviously going to take time to feel comfortable in the role, but for a first game in that position, there were some really positive signs," Watters said.
"That won't preclude him from playing at the other end of the ground, either, but at this stage we'd like to settle him in our back six.
"Obviously we'd like to strengthen that area from a height point of view.
"He has great characteristics of a defender - good speed, very good endurance and he's 200cm.
"There's not many of them around."
Stanley has played 21 games for the Saints since his 2010 debut as a key forward or back-up ruckman.
He suffered two serious hamstring injuries last year, which restricted him to 12 senior games.
"I told him I was glad it wasn't his `hammy'," Watters said post-match.