The club captain, 32, is in contention to start with Laurent Koscielny suspended, Gabriel Paulista injured and Shkodran Mustafi battling to overcome illness.
Should Mertesacker start he would have to adjust to Arsenal's new 3-4-3 formation on only his second appearance of a season disrupted by a knee injury.
While he may have recovered, there remain significant concerns over his match fitness against the energetic Premier League champions, who scored 85 times in the process of securing a record 30 league victories, particularly given the potential for extra-time.
Asked if he would be ready if selected by manager Arsene Wenger, Mertesacker responded: "I cannot tell you.
"I haven't played this year. 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 this to this team and to this club.
"I want to make sure I will be ready and my mindset is '120 minutes plus penalties'.
"(It is) completely unexpected. I made the squad because of two injuries and then I came on (on Sunday in the 3-1 defeat of Everton) due to one red card and one injury.
"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."
There also remains uncertainty surrounding Wenger's future and whether Saturday's fixture at Wembley could be his last match as manager after 21 years at the club.
Victory would give him a record seventh FA Cup, but asked if he felt this could be Wenger's last game, Mertesacker said: "I have absolutely no idea. I cannot tell.
"First of all we are lucky to have him here. That was always my mindset when I came here. I was so happy to have him. He trusted me a lot.
"I can just tell you I'm looking forward for the future of this club because even if we struggle a bit, or play in the Europa League next year, it gives us a little bit of time and a breather to think about what has gone wrong last season and we do not deserve to be in the Champions League.
"Short-term, we are just thrilled to be in that final with our manager."
Mertesacker has not started a game in almost 13 months and Wenger admitted selecting the German might depend on the formation Arsenal play.
"I don't know as he has not played 90 minutes for a long time," Wenger said.
"I have not made up my mind yet. He has the leadership qualities so it depends a little bit on the system I will go for, the system I will choose."
Wenger will also make a late decision on Kieran Gibbs, who was yet to return to training on Wednesday as the full-back tries to overcome a thigh injury.
