Tottenham's first game of 2020 is taking place at St Mary's on New Year's Day.
Jose Mourinho's side were looking to get the new decade off to a winning start, but fell behind to a Danny Ings goal when the Saints striker split the central defensive pairing of Jan Vertonghen and Toby Alderweireld to fire past Paulo Gazzaniga.
With so many games in a short space of time over the festive period, the Portuguese took the opportunity to make some changes to his starting lineup.
One of those changes came at left-back, where Ryan Sessegnon was brought in to replace Vertonghen, with the Belgian moving into a more familiar central defensive role alongside international team-mate Alderweireld.
Sessegnon has played at both left-back and left wing for Fulham before signing for Tottenham in the summer and has already played in both positions for Spurs.
He has been preferred in a more advanced role in N17 so far, but Mourinho decided to give him a chance in the back-line against Southampton, with no Danny Rose or Ben Davies available due to injuries.
And Mourinho explained his decision to play the youngster before kick-off.
He said: "We know how he is, what he can do. But in this moment we have no Danny Rose, no Ben Davies, it is either Ryan or Jan Vertonghen and today we decide to go with Ryan."
Sessegnon has struggled to get forward from his left-back position in the first half, with Southampton pushing him back.
