Manchester United have given Marcos Rojo permission to train with Estudiantes, the Argentine club have said.
The 29-year-old last played for United in the 2-1 Champions League defeat away at Valencia in December and has featured only three times in total this season.
Chris Smalling and Victor Lindelof have become United's first choice centre-back pairing since Ole Gunnar Solskjaer replaced Jose Mourinho in December.
An Estudiantes tweet read: "Nothing beats spending your birthday at home! Marcos Rojo was present at City Bell to be able to train and stay in shape with a special permission of @ManUtd."
Rojo has over two years remaining on his contract at United, who have the option to extend his deal by a further year.