Unai Emery sent ex-Valencia defender Miguel Brito to sleep during a tactical meeting while in charge at the Mestalla, according to Dani Parejo.
Arsenal have arrived in Spain for a huge Europa League semi-final second leg showdown on Thursday, boasting a 3-1 lead from the reverse fixture.
An Alexandre Lacazette brace and a late Pierre Emerick Aubameyang effort put the Gunners in pole position to qualify for the final, but Mouctar Diakhaby's early strike gave Valencia a crucial away goal.
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Emery is pitting his wits against the club he managed between 2008 and 2012, which was his first major role as a head coach.