Romeu came up against his former employers for the first time when Ronald Koeman’s side won 3-1 at Stamford Bridge earlier in the season.
Goals from Steven Davis, Sadio Mané and Graziano Pellè sealed the win that day, but Romeu is expecting a different test when a rejuvenated Chelsea visit this weekend.
“It was a big win and the feeling that gave the team was very good,” he told reporters ahead of the game. “We took a lot of confidence from that result.
“They had different feelings back then and weren’t playing at their best, but now it’s going to be a different test.
“I would never have said they would be where they are now. It’s incredible that they are in this position, but that just shows how difficult this league is.
“It’s what can happen if you’re not at the top of your game.”
Romeu spent four years on the books of Chelsea before making the move to St Mary’s in the summer, and while he admits it was strange coming up against them for the first time, he has no regrets about joining Saints.
“There were good times and some difficult times [at Chelsea],” he added. “Everyone knows that I had to go through a difficult injury, but I got better with the help of some great physios.
“It was a good family and obviously it was great to be at the club. I wanted to continue playing in the Premier League.
“I wanted to keep going and for me Southampton was a very good club. The way they were playing was very good and I’m very pleased with my decision.
“[The last game] was a chance to catch up with a few friends but I was there to do my very best like every other game because I want to get this club as high as I can.”