The midfielder was due to start the Bundesliga side's pre-season friendly against VfB Oldenburg on Sunday.
However, he was replaced by Nick Woltemade shortly before kick-off after picking up a knock in the warm-up.
It's not clear what the nature of the knock is or how long it could keep Keita sidelined for.
But it's yet another setback for the 28-year-old, who was dogged by knocks throughout his five years playing for Liverpool.
While he was part of Jurgen Klopp's squad that won the Premier League, Champions League, FA Cup, Carabao Cup, Club World Cup, Super Cup and Community Shield, Keita's time on Merseyside will be remembered for the many absences he had with injury.
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Along with James Milner, Roberto Firmino and Alex Oxlade-Chamberlain, Keita's departure from Liverpool was announced in May upon the expiration of his contract. His free transfer to Werder Bremen was confirmed in June.
Liverpool struck a deal to sign Keita from RB Leipzig in August 2017 in a £48million deal but the Guinea international spent the 2017/18 season with the German outfit before making his debut for Liverpool in August 2018.
Keita represented Leipzig for two years having joined them from Austrian side RB Salzburg. He has 54 caps for the Guinea national team.
Werder Bremen ran out 3-1 winners on Sunday thanks to goals from Justin Njinmah, Dawid Kownacki and Jean-Manuel Mbom.