The Gunners, along with Manchester United, Everton, Newcastle and Sheffield Wednesday had scouts watching the Portuguese league champions in action.
Record do not say who Arsenal may have been watching, but the likeliest candidate would have been winger Andrija Zivkovic who the Gunners have been linked with over the past few months.
The 21 year old replaced Alejandro Grimaldo on the hour mark, and went on to the score the sixth goal three minutes from time in a one sided contest. The Serbian has had to settle for cameo appearances in the last two games since the international break, after also being used as a substitute in the 2-0 defeat to CSKA Moscow last week.
Zivkovic moved to Portugal from Partizan Belgrade on a free transfer in July 2016 but many European clubs have been monitoring the young winger’s development over the past 18 months.