Brazil midfielder Diego is back at Wolfsburg but didn't take part in Tuesday's training at the Bundesliga club.
The 27-year-old was recently in his homeland for treatment for a knee injury.
His future is still uncertain, despite having a contract with Wolfsburg until 2014.
Diego was on loan to Atletico Madrid last season.
He fell out with Wolfsburg coach Felix Magath late in the 2010-2011 season.
Flamengo and other clubs were reportedly after Diego but the possible deal with Flamengo failed for financial reasons.
Newly-promoted Hertha Berlin has strengthened its squad for next season's Bundesliga campaign by signing Japan midfielder Hajime Hosogai from Bayer Leverkusen for one million euros ($1.31 million).