Mikel Obi was in full-time action for Chelsea, as the Blues slumped to a 1-0 loss away to Swansea in South Wales on Saturday.
The defeat was its 11th of the season but the first in the Premier League under Guus Hiddink.
Jefferson Montero’s cross was headed away by the Chelsea defence as far as Sigurdsson who volleyed home first time.
Montero danced his way past CesarAzpilicueta and a couple of defenders before dinking in a cross which Matt Miazga poorly cleared to the path of the Icelandic international to hand the hosts the lead.
Alexandre Pato had a clear-cut chance to restore parity for the Blues but the petit Brazilian forward fired his shot wide from the box.
At the start of the second half, Guus Hiddink pulled off Miazga for Kenedy with Mikel dropping to the centre half role.
As the second half wore on Chelsea failed to carve out any scoring chance to get back into the game.
Falcao came on for Reuben Loftus-Cheek as Chelsea sought for an equalizer but all to no avail.
The result sees Swansea move up to 13th on the log while Chelsea remains tenth on the log.