Arsenal defenders Sead Kolasinac and Nacho Monreal will both miss Arsenal's trip to Sporting CP in the Europa League on Thursday.
With Arsenal looking to win an 11th match in a row in all competitions, following their 3-1 victory over Leicester on Monday, the news comes as a blow to Unai Emery who has no more natural cover in the position.
Both men have been suffering with hamstring strains - as a result the duo have been left at home instead of traveling to Portugal with the rest of the squad.
Ainsley Maitland-Niles is one player who could have potential filled in at left back but he is also injured, and has not played since the opening game of the season.
However, the injuries may lead a path back into first team football for forgotten man Carl Jenkinson, who has not featured for the Gunners since November 2016.
Stephan Lichtsteiner was used in the left back position against Leicester on Monday night and if Emery wants to start with the Swiss man again, and rest first team regular Hector Bellerin, Jenkinson could quite possibly play in the right back role.
Sporting, like Arsenal, have won both of their games in Group E so far, and this will most likely be the toughest test Arsenal face in the group stages of the competition.
As reported by Sky Sports, Unai Emery talked up the importance of the competition and the quality of Sporting CP.
The three-time Europa league winning manager said: "For us this competition is very important.
"Sporting are a very good team, with big experience and good players. In this moment we are two teams both in first position. In our mind we want to be first in this group.
"It is a very big match and very important for our confidence."