Bayern Munich had planned to unveil their new signing Serdar Tasci today.
The 28-year-old joined the Bavarians from Spartak Moscow earlier this week on a six-month loan deal.
However, Tasci was not present at the press conference due to a head injury that he suffered in training.
"He had a collision and in the aftermath showed symptoms. It indicates he suffers from a minor concussion," Bayern's sporting executive Matthias Sammer told reporters.
The centre-back is now regarded as doubtful for the weekend's Bundesliga clash against Bayer Leverkusen.
