Casillas, who made his Champions League debut as a tender 18-year-old in 1999, holds the competition record with 170 appearances spread over a remarkable 19 seasons.
"San Iker", the captain of Spain's World Cup-winning side in 2010 and of back-to-back European Championship triumphs, now finds himself Porto's back-up ahead of Wednesday's last-16 clash against Liverpool.
Casillas, who made his Champions League debut as a tender 18-year-old in 1999, holds the competition record with 170 appearances spread over a remarkable 19 seasons. But an apparent lack of application in training prompted his coach to replace Casillas, who is now 36, as Porto's first-choice goalkeeper.
While away to RB Leipzig during the group stage in mid-October, Porto boss Sergio Conceicao dropped the veteran Spaniard -- a European champion with Real in 2000, 2002 and 2014 -- in favour of 25-year-old Portuguese deputy Jose Sa.
While the baptism of fire did Sa few favours as Porto slid to a 3-2 defeat in Germany, it symbolised the passing of the torch as Sa went on to establish himself in the club's number one jersey.
The change to the hierarchy led former Portugal international Conceicao to clarify the situation during a press conference, where he pointed the finger at Casillas for a lack of drive and energy during the team's daily training sessions.
"With Iker, it's not a disciplinary problem. I demand all of my players to always be at 100 percent at training and I can't close my eyes to the fortnight of training that didn't meet my demands. Iker understood it," Conceicao explained.
Far from engaging in a stand-off with his coach, as he did during Jose Mourinho's tenure at Real, Casillas doubled down and ruled out any move away from the club in January.
"Everyone drops to the bench one day, there are not only 11 players. I like to train, push myself and fight for a starting place," the Madrid-born Casillas said in December.
While there is no guarantee the Spaniard will regain the first-choice role at Porto, he can at least draw some comfort from a series of promising run-outs in the two domestic cup competitions.
Casillas was unable to prevent Porto's penalty-shootout defeat to Sporting in the League Cup last month, but he gave an assured display to help secure a 1-0 advantage over the Lisbon outfit in the semi-finals of the Portuguese Cup a week ago.
That match marked the 100th in Porto colours for Casillas, and demonstrated the 36-year-old still has the ability required to shine at the top level.
Back in Conceicao's good graces, the Porto coach even hinted recently that Casillas could be in line for a return to the leading role between the posts.
"I have four fantastic goalkeepers and Iker has worked really well these past weeks so I won't reveal who I'm going to start, it's the position where I have the most uncertainty," Conceicao said.