The Paisley outfit initially feared the Scotland Under-21 international had suffered a cruciate ligament injury following a training incident but scans have now confirmed the player will only be sidelined for six weeks.
The Scottish Communities League Cup winners' medical team said McLean had suffered bone bruising and some ligament damage but are confident they player will be back by early September.
St Mirren manager Danny Lennon said: "As always our first priority is to the player and helping him back to full fitness.
"When the injury first happened I thought Kenny had taken a really bad one that was going to put him out of the game for a lengthy spell, but thankfully it wasn't as bad as first feared and all being well he will be back in action in six weeks or so.
"Kenny's been looking really good over the pre-season and we have been expecting big things of him over the upcoming season so it is a big blow to both the team and Kenny.
"But the injury isn't as bad as first feared as he will be back in less than two months when I'm sure he'll come back stronger and ready to go."