With just 5 minutes gone, Etoo got on the end of an outstanding through ball slipped past the Arsenal defence, and the former Inter and Barca man cut back on to his left foot and curled an excellent strike beyond the flailing Wojciech Szczesny.
The Chelsea striker was only on the pitch for the first 10 minutes, with what looked like a hamstring injury seeing him replaced by Fernando Torres.
The Blues managed to scored a second before he left proceedings, Andre Schurrle scoring a fantastic drilled effort into the same corner Etoo found.
In an absolutely bizarre first half in London, referee Andre Mariner would go on to give a penalty and red card against Arsenal following Alex Oxlade-Chamberlain's handball, somehow sending off Kieran Gibbs rather than the former Southampton man. Eden Hazard scored the resulting penalty to make it three, with Oscar getting a fourth just before half time.