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.
Emery is pitting his wits against the club he managed between 2008 and 2012, which was his first major role as a head coach.
And ahead of the return leg, Valencia star Parejo described the Spanish coach as a "very good coach who's done incredible things", but also recalled an incident which underlined his meticulous approach behind the scenes.
The 30-year-old claims a drawn-out meeting between the club's squad and Emery at their training ground left Miguel Brito in a deep slumber, as he told The Guardian: "They say coaches are a little different, and he's a perfectionist: there's a lot of work, a lot of videos, talks, and its true players can get tired of it, that he's a bit heavy-going.
"When you get an outline of the opponents it helps, of course, but a footballer's attention - anybody's attention - can wane if it goes on for 20, 25 minutes.
"Miguel Brito fell asleep during a talk one day."
Emery's first year at Emirates Stadium has garnered mixed results, with his side currently occupying fifth place in the Premier League table.
With only one round of fixtures remaining, the Gunners must beat Burnley at Turf Moor this Sunday, while hoping Tottenham suffer a defeat at home to Everton and for there to be an eight-goal swing in terms of goal difference.