Arsenal’s star winger Bukayo Saka faces an injury concern following the Gunners’ 1-0 victory over Tottenham in the north London derby ahead of crunch fixtures in Europe and the Premier League. Manager Mikel Arteta expressed uncertainty about Saka’s condition after the player asked to be substituted in the 86th minute. “Suddenly he went down and said he couldn’t continue and we had to take him off,” Arteta told Sky Sports. The Spaniard later added in a post-match press conference: “I don’t know. I made two changes with the front players and suddenly he’s down. “I don’t know exactly what it is b…