With Virgil van Dijk sidelined with an ankle injury, Saints opted to bring in former Barcelona and Juventus defender Cáceres on a free transfer earlier this month.
The 29-year-old has not played a competitive game in over 12 months but having impressed his new manager during the club's recent training camp in Spain, he could be handed a baptism of fire against Jose Mourinho's side at Wembley.
"It is a good surprise about Martin, because he is physically fit and we will see if he can participate for the final, because it would be his first game in one year," Puel told the club's official website. "But I think he can give a good solution – perhaps for the final, and for the future."
Sofiane Boufal is also in contention to feature on Sunday. The Morocco international suffered an ankle injury in the 3-1 defeat to West Ham on 4 February and missed the 4-0 thrashing of Sunderland but returned to training this week.
Puel was wary of confirming whether or not the club's record signing would start against United, with the club to make a final decision after further assessments on Saturday.
"Sofiane comes back step-by-step in the training sessions," said Puel. "We will see tomorrow. He is better and I hope he can participate also with the squad."
In addition to Van Dijk, Southampton also remain without striker Charlie Austin, defenders Jeremy Pied, Matty Target and goalkeeper Alex McCarthy.
United meanwhile will be without Henrikh Mkhitaryan, with the Armenia international ruled out after suffering a hamstring injury against Saint-Etienne on Wednesday. Michael Carrick also limped off in that match, but the veteran midfielder will be available for Sunday's fixture.
Wayne Rooney and Phil Jones are also in contention after recovering from knocks.