The Portuguese side held the Reds to a draw at Anfield on Tuesday, sealing their elimination from Europe's premier club competition n midweek after having lost the first leg 5-0.
Casillas, 36, will be a free agent at the end of the season when his contract expires, meaning he may not get another chance to win a fourth Champions League crown in his career.
However, in shutting Jurgen Klopp's side out in Merseyside, Casillas feels he can be proud of his potentially final European game.
"If this was my Champions League farewell, it hasn't been a bad night at all - at this fantastic stadium, keeping a clean sheet... but no one knows what the future holds," he told reporters.
"If I look back, I can feel proud of the things I've won in the game, of everything I have achieved. I would like to thank the fans. It's been many years in the Champions League and it's great to come to a stadium like this and be given such a warm reception.
The World Cup winner has the rest of the season to see out with Porto and is targeting a Primeira Liga title to close his career, with his side sitting five points clear of Benfica after 25 matches.
"My objective is to win the league with Porto because this group of players deserves to lift the trophy," he added.
"We'll see what happens at the end of the season."