Cahill missed the FA Cup semi-final win over Tottenham Hotspur with gastroenteritis and was replaced by Ake but the England centre-back starts against the Saints.
Hazard and Costa were both benched against Spurs but came on, with the former shining in particular, but they start in attack.
Willian, who scored twice against Spurs, drops to the bench along with Pedro and Michy Batshuayi, as Cesc Fabregas also comes into the team.
The Spaniard could play centrally, with Hazard closer to Costa, or he could play wider on the right.
Southampton name a strong team with Sofiane Boufal and Manolo Gabbiadini both starting in attack along with Dusan Tadic.
Former Blues Ryan Bertrand and Oriol Romeu both start.
Read the full article on eurosport.co.uk: Team news - Cahill, Hazard and Costa return, Willian benched