The Frenchman hobbled off the Emirates turf in the 21st minute and was replaced by Rob Holding.
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The Gunners had just taken the lead in the last 16 tie through Granit Xhaka, having drawn the first leg in Lisbon 2-2.
But there was instantly some bad news as Saliba left the game with what appeared to be a back injury, joining fellow defender Takehiro Tomiyasu in being a first-half casualty.
Only Bukayo Saka and Gabriel Magalhaes have played more minutes for Mikel Arteta in all competitions this season, with Saliba a key piece of their Premier League-leading form.
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It could be a significant moment as Arsenal look to bring home a first Premier League title in 19 years.
Arsenal face Crystal Palace at the Emirates on Sunday before the international break and still need to face Liverpool, Manchester City, Chelsea and Newcastle in their run-in.
Catch the latest updates from Arsenal vs Sporting on our live blog here.
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