Trundle, who only signed for North End this summer, suffered a cartilage tear in the pre-season friendly loss to Morecambe 10 days ago and he will undergo surgery this week.
The news comes just days after chairman Peter Ridsdale revealed the club were chasing the signature of an unnamed striker.
Manager Graham Westley told the Lancashire Evening Post: "The urgency to finish off the squad and build the top end of the team has been there anyway.
"He's [Trundle] damaged both sides of the meniscus and strained the medial and cruciate ligaments. He has never had a serious operation in his career and is obviously gutted to be in this position.
"The strains are nothing to worry about - it's the cartilage which is the main damage and they are going to stitch it back together rather than pull it all out. It's probably going to be three or four months before he's back."
- Peter Ridsdale