Leeds United midfielder facing two weeks out with quad injury
Dallas was brought on as a 76th-minute substitute for Mateusz Klich in centre-midfield during Sunday's 3-1 victory against Stoke City at Elland Road but the 27-year-old has since picked up a quad injury and will play no part in Saturday's Championship clash at Derby County.
Dallas joins midfielder Adam Forshaw on the sidelines with head coach Marcelo Bielsa otherwise having a full squad to pick from for the weekend's showdown at Pride Park.
Forshaw has damaged ligaments in his foot with the 26-year-old looking at around six to eight weeks out and Bielsa has suggested that he could start with the same XI at Derby that set out against Stoke.
"If everything goes well it will be the same team and we won't be able to use Dallas," said Bielsa.
"He has an injury and he won't be available for a certain period.
"Dallas can play on both sides at different levels. So the fact that he won't be with us for a while is something we regret."
Bielsa also hinted that centre-back Pontus Jansson could play a part at Pride Park with the 27-year-old continually gaining in fitness after making a late return to Thorp Arch following his exploits with Sweden in the World Cup.
"He is improving every day," said Bielsa.
"He is getting closer to his best fitness and condition. And he is a possibility to be a right central back or left central back."