The midfielder joined the Spanish giants in 2009 after three years with the Premier League side.
The 30-year-old was loaned out three times while at Barca as he failed to secure a regular place in the team.
"I'm sad to have lost the best years of my career, something I only blame myself for," he told Stuttgarter Zeitung.
"Over time I understood that the force of Barca was the ability of players to leave their ego aside and work for the good of the group."
Hleb recently spent a loan spell at Birmingham City.