Wayne Bridge is out for the rest of the season with a suspected broken left ankle.
Bridge suffered the injury as a result of a challenge by former England striker Alan Shearer in the disastrous FA Cup defeat at St James' Park on Sunday.
While it is too early for the medical team to confirm the extent of the injury to Bridge, Chelsea boss Jose Mourinho has already discounted the player for the rest of the season.
He said: "We think it's a big injury. He has no chance of playing against Barcelona, probably will not play against Liverpool (in the Carling Cup final) and maybe not for the rest of the season."
With Bridge out and Celestine Babayro sold to Newcastle, Chelsea do not have a recognised left-back.
Gallas has played there when Bridge was out of favour, but he is not a natural left sided player.
A crisis is looming with two massive games coming up within the space of four days.
After the Champions League encounter in Spain, Chelsea head straight into preparations for Sunday's Carling Cup Final against Liverpool in Cardiff.