The English forward jumped for a header with Bruno Gaspar in the first half and came down awkwardly.
Sporting ended the game with 10 men when ex-Barcelona defender Jeremy Mathieu was shown a straight red card after fouling Pierre-Emerick Aubameyang, who was clean through, although the hosts could not find a winning goal.
Chelsea made it four wins from four matches in Group L by edging out a spirited BATE Borisov 1-0 in Belarus.
"He tells you exactly what he wants you to do, but gives you freedom to expose your talent".
The Londoners moved six points clear of Hungarians Vidi at the top of the group and have not tasted defeat in 17 matches since losing the Community Shield to Manchester City in August.
The Gunners have been impressive in their Europa League group so far and have won all three of their group games, meaning victory tonight at the Emirates Stadium would book a last 32 spot as group winners.
Salzburg, semi-finalists last season, qualified for the next round with a 5-2 thrashing of Rosenborg in the group´s other match, as Japanese forward Takumi Minamino scored a hat-trick.
Numerous fans had differing reactions to how the game went on social media, with some just pleased that they have qualified for the latter stages, whilst others were disappointed with the performances from some of their players.