In a court filing Thursday, the lawyers say Cardinal Anthony Bevilacqua didn't recognize Monsignor William Lynn at the cardinal's deposition last week.
The 88-year-old Bevilacqua suffers from dementia, although a judge found him competent to testify.
Lynn, the secretary for clergy from 1992 to 2004, is charged with conspiracy and child endangerment for allegedly transferring predator priests. Three priests and a former teacher are charged with raping boys.
Prosecutors want to preserve Bevilacqua's testimony before the March trial.
But Lynn's lawyers say he had little recall of relevant events at the two-day deposition. They have argued that Bevilacqua, not Lynn, made transfer decisions.