The creative midfielder has been out of action for five months and his progress remains unclear, with the sporting director admitting it is "impossible" to gauge a return date
The Spain international last featured for Bayern in the 3-3 Bundesliga draw with Hoffenheim back in March and has been out of action ever since due to a knee injury.
Thiago was initially expected to be match fit in time for the start of the 2014-15 campaign, but the 23-year-old is still recovering from his knee problem, much to the frustration of Bayern.
"We cannot really say how long his recovery will take," Sammer was quoted as saying by Kicker.
"It's impossible to set a date for a comeback. This is a very regrettable development."
Thiago joined Bayern from Barcelona in the summer of 2013, but injuries have so far prevented him from making an impact at the Bundesliga champions. He made 25 appearances in all competitions for Bayern last term, scoring three goals in the process.
The German champions are still without injured pair Javi Martinez and Bastian Schweinsteiger, too.