The company wrote in a blog that executives determined the signatures weren't authentic after "extensive research and working closely with McKinley's representatives."
Company officials declined to comment about further specifics. Executives with McKinley's current representation, Rep1 Sports, did not immediately return messages seeking comment.
Since the autograph cards were inserted into two sets that were already released, collectors can send McKinley cards back to Panini in exchange for an autograph the company guarantees will be authentic.
Autographed McKinley cards from the product that are selling on eBay all seem to feature the same signature for McKinley -- a printed "Takk."
Brian Gray, whose company, Leaf, also has McKinley autographs signed in a similar manner, told ESPN on Monday he has been assured that autographs signed for Leaf are authentic.
Companies like Panini and Leaf pay draft picks per signature -- an industry source says McKinley's rate was in the $2 to $5 range -- and the players sign sticker sheets that are then added onto the cards.
Players have in the past had other people sign their cards, which has led to the companies scrutinizing the signatures to a greater extent.
Gray said he once had a player run the sheets through a color printer so that it would print out the autograph perfectly.
Said Gray: "They looked great. The only problem was that they all ran off the page in the exact same spot, so we knew they weren't real."