Casillas has not been enjoying a prosperous spell at Porto as of late, having been relegated to the bench in recent weeks in place of the promising Jose Sa.
It's been reported by Alfredo Relano of AS, the issue for the veteran is the salary he's picking up, with the Portuguese side said to be concerned about meeting financial fair play guidelines.
"Porto are taking great care to comply with financial fair play, and Casillas has a decisive weight as the club look to square off their accounts," he noted. "And if, as appears to the case, in Sa they have a fine goalkeeper for the next 10 years, why spend so much money on Casillas?"
As noted by Dermot Corrigan of ESPN FC, Casillas' former club Real Madrid have paid part of the goalkeeper's salary in previous seasons:
All these factors appear to add up to a potential January transfer for the Spaniard, who is regarded as one of the finest goalkeepers of his generation.
Casillas will feel as though he still has something to bring to the highest level. After struggling for his best form at Porto following his move in 2015, he settled in to life at his new club and prior to his benching was enjoying his best spell in Portugal.
As relayed by WhoScored.com, the legendary goalkeeper helped contribute to Porto's strong defensive record as of late:
Casillas adds much more than goalkeeping skill, though. In his 16 years as a senior player at Real Madrid, he developed a winning mentality and brilliant leadership traits.
Factor in his displays for Spain in their two UEFA European Championship wins and their 2010 FIFA World Cup win, and Casillas has accomplished an incredible amount in the game.
He's won the UEFA Champions League three times and is a legendary figure in the competition's history, per the tournament Twitter account:
Liverpool would benefit from a new goalkeeper, as Simon Mignolet continues to make errors. As noted by Oliver Bond of Anfield HQ, they're mistakes that tend to cost the Reds:
Given there are doubts over potential replacements for Mignolet in Loris Karius and Danny Ward, Klopp may view Casillas as a short-term solution for the problems at the base of the team.
Yet this is an issue that should be looked at with a long-term view, and while the Spaniard could do a decent job until the end of the season, by the summer, thoughts will begin turning to who should replace him. With that in mind, Casillas and the Reds don't feel like a natural fit.