The 29-year-old former Barcelona and Juventus defender joined the club last week and impressed boss Claude Puel during a training in Spain.
United captain Wayne Rooney, who had been linked with a move to China, will be involved, said boss Jose Mourinho.
Mourinho is looking to add to the three League Cups he won at Chelsea.
Victory in Sunday’s match at Wembley would mean the Portuguese wins a major trophy in his first season as United boss.
“The important thing is the club and obviously the club won a trophy last season,” he said, referencing the Red Devils’ FA Cup success.“Let’s try to win another one this season.“I am in a period when I am more and more and more a club man.”
Team News-
United attacking midfielder Henrikh Mkhitaryan will miss the game after injuring a hamstring during Wednesday’s 1-0 Europa League win at Saint-Etienne.
Midfielder Michael Carrick is fit despite injuring a calf in France, while a late decision will be made on defender Phil Jones, who has a foot problem.
Southampton hope playmaker Sofiane Boufal will return from an ankle injury.
Charlie Austin, Virgil van Dijk, Jeremy Pied, Matt Targett and Alex McCarthy all have long-term injuries.
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