Chelsea suffered a narrow 1-0 defeat to bottom-of-the-table Southampton in the Premier League on Saturday, piling more pressure on manager Graham Potter.
Southampton's James Ward-Prowse scored a free-kick just before half-time which proved enough to edge his relegation-threatened side to victory at Stamford Bridge.
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Azpilicueta needed nearly 10 minutes of treatment on the pitch in the second half after he received a kick to the head in a goalmouth clash. Physios from both clubs rushed to his aid and the Spaniard was eventually carried off on a stretcher.
Chelsea looked livelier in the second half when Raheem Sterling and Kai Havertz came on and they kept the Southampton defence under pressure.
Southampton goalkeeper Gavin Bazunu was kept busy and in the dying seconds of added time produced a fingertip save from substitute Conor Gallagher.
The result -- leaving Chelsea still with just a single victory in 10 games this calendar year -- puts an end to a poor week for the west London club that also saw them fall to a late away defeat to Borussia Dortmund in the first leg of their Champions League round of 16 tie.
Chelsea are languishing in 10th place on 31 points from 23 games and are 10 points adrift of the top four.
Potter left Brighton to replace Thomas Tuchel in September but has struggled to revive Chelsea's fortunes, despite spending heavily in the January transfer window.
Meanwhile, Southampton remain in last place but move to three points from safety and are led by caretaker coach Ruben Selles after Nathan Jones was sacked a week ago.
"It has been a tough season for us," Ward-Prowse told the BBC. "We have lacked a bit of consistency in all aspects of our game. Ruben has come in this week and been fantastic. He has steadied the ship and calmed everyone down."