Southampton broke their club record transfer fee on Sunday when they paid Roma £15 million (17.6 million euros) for striker Pablo Osvaldo.
The deal for the 27-year-old Argentina-born Italian international betters the £12.5million Southampton paid for Celtic midfielder Victor Wanyama in July.
He will now be reunited with Saints boss Mauricio Pochettino under whom he played during an 18-month spell with Spanish side Espanyol.
Osvaldo, who had also played in Serie A for Atlanta and Fiorentina, should be available for Saturday's Premier League game against Sunderland at St Mary's.
