The 32-year-old now looks set to start at Wembley with fellow defenders Laurent Koscielny (suspended) and Gabriel Paulista (knee injury) out of the final, while Shkodran Mustafi (concussion) and Kieran Gibbs (thigh injury) are major doubts.
"It is completely unexpected... " Mertesacker told reporters on Wednesday.
"It was unexpected but you need to be ready at any time. It was good to get a run-out and obviously make my first appearance of the season after a long, long injury. I was very grateful I got the opportunity."
The German is confident that his experience can help him excel despite his lengthy spell on the sidelines as Arsenal look to win the FA Cup for a third time in four years.
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"I haven't played this year," Mertesacker said. "But I have done this for 15 years so I expect myself to be absolutely ready no matter what comes.
"That is my mindset, no matter what happens or what is facing me on Saturday, I am going to go for it. I owe it to this team and to this club. I want to make sure I will be ready and my mindset is '120 minutes plus penalties'."
Arsenal, who finished fifth in the league, extended Mertesacker's contract in January.
(Reporting by Aditi Prakash in Bengaluru)
