But at the age of 32, the La Masia academy graduate made a sensational return to Camp Nou, helping his side to the competition's quarter-finals.
Romeu made seven appearances against Europe's elite and played a further 30 times in domestic competitions, finishing second in La Liga.
The midfielder left Barcelona without breaking into the team in 2011, joining Chelsea, and could never have predicted he would return 12 years later.
His performances for newly-promoted Girona, in the season that he was sold by Saints, earned him that privilege, playing nearly 3,000 La Liga minutes.
"I was very lucky to be there," Romeu told the Daily Echo. "My first season at Girona was good. We finished top 10 and I played all the games.
Oriol Romeu returns to Barcelona after starting his career in La Masia 19 years ago 🔵🔴 #FCB
◉ Contract until June 2026.
◉ Documents signed with Girona.
◉ Release clause into contract: €400m.
Official, confirmed. ✔️ pic.twitter.com/zHnzkYuYkn
— Fabrizio Romano (@FabrizioRomano) July 19, 2023
"I felt really good. That's why it allowed me to sign for Barca the year after, and it was great. I started playing almost everything by Christmas.
"After Christmas, I didn't play that much, and I was coming on mainly as a substitute, but overall it was an amazing experience, I have to say.
"Playing with those players, also playing in the Champions League, the Super Cup, all these amazing games, the quarter-finals in Europe.
"It is something that you never expect at 30 years old, but that chance comes to you again and you're very pleased that you can live it out of nowhere.
"But I'm not thinking that because I played Champions League last season, I should now play easily in the Championship - I know how tough it is."
Romeu switches focus straight back to business at home again, after signing a short-term deal to return to St Mary's three years after his departure.
He could make his second debut for the club at Charlton on Saturday, but is already eyeing up a second visit to Portsmouth's Fratton Park in January.
"I remember when I was here, and although they were in a different situation and we were in a different situation, it was the atmosphere," he said.
"The game was one of the highlights of these seven years here, so I am willing to go there again and hoping to beat them, not that easy probably.
"But we are hoping to beat them again and make sure that this win allows us to be in a higher position and to fight for promotion, which is what we want."