Chelsea interim manager Guus Hiddink says his side is wary of the threat that Southampton poses when the two teams meet on Saturday.
The Saints have struggled for form in the first part of the season but their recent impressive run saw them move up to sixth in the table.
The Dutchman also confirmed that captain John Terry is still on the sidelines as he continues his recovery from injury.
"After a little setback they [Southampton] have regained their confidence in recent games," Hiddink said.
"They can maintain their clean sheet, but also they can play and surprise their opponent. They have a mix of young players but also players who know the game. That makes them very dangerous.
"It will be a very difficult game getting them out of their clean-sheet comfort zone. It's up to us to try to annoy them.
"It will be a big test for Chelsea to see where we are in the Premier League.
"He [John Terry] will not be fit. He is still in treatment, every now and then he goes in the gym. He is doing exercises there and on the pitch but he is not fit.
"We want to be careful with this kind of injury.
"When you have something in the tendon or in the hamstring then sometimes you think, as a player or manager, that it appears to be good, but you must be careful.
"I am in favour of waiting two or three days more because it's not good to have a setback. We have Norwich on Tuesday and we will see whether he will be available and fit."