The Saints lost their second successive game in the Premier League, as they went down 2-0 to Tottenham at home.
England international striker Harry Kane put Spurs in the lead in the 40th minute, and three minutes later young midfielder Dele Alli made it 2-0.
Mills was not impressed with the way Southampton defended during Kane’s goal and said on BBC Final Score: "Brilliant finish from Harry Kane, once he got through.
“But Southampton's defending was comical - Kane ran through three challenges with no-one getting any contact on the ball or the man.”
Southampton were poor against Tottenham and deserved to lose the game. Ronald Koeman’s side have not played well in recent weeks, and defensively they need to improve.
The Saints have conceded as many as 21 goals in 17 Premier League games so far this season. For a team that should be aiming for a European place, that is not good enough.
Southampton are 12th in the Premier League table at the moment with 21 points from 17 matches.