Kelleher is Liverpool's designated goalkeeper for the competition.
However, Alisson started the first leg - which ended in a 0-0 draw - and Kelleher says he'll be ready whatever happens.
"I'll just prepare like I am [playing] but I'm not fully sure yet," Kelleher told Liverpoolfc.com on Tuesday.
"We had a little chat with the manager [before the first leg] and he just explained that Alisson had a few weeks off with COVID and stuff, so I think he needed the game just to get back in the rhythm of things after being out for a while. I fully understood the reasons."