Saints have lost a number of key players over the last few seasons, but they were able to avoid a repeat of a mass exodus this time around, despite Virgil van Dijk's desire to depart St Mary's.
The Dutch international did not get his wish despite reported interest from Liverpool, while fellow defenders Ryan Bertrand and Cedric Soares have also remained on the South Coast amidst speculation linking them with moves.
Defensive midfielder Romeu believes it was crucial Saints were able to keep the core of their squad together and is convinced it will benefit them over the course of the season.
"It's very important to keep the structure of the team we had last season," Romeu said. "We felt good, we felt we could play good football and the longer you play with someone the better you feel playing next to him."
Despite keeping hold of their key players, Southampton do have a new manager at the helm in the form of Mauricio Pellegrino and Romeu believes the players are learning all the time from the former Alaves boss.
The 25-year-old added: "Now we have a new manager, new things to learn, but the team is more or less the same. I truly believe that will be better for us."