Moussa Diaby, the record £42 million signing, was sent into space in the area by Bailey and his low shot beat Tzolakis at the near post. Villa had been in the ascendancy but the game swung back towards the visitors three minutes later.
After Villa failed to clear a corner, Panagiotis Retsos headed the ball back towards goal and Douglas Luiz clearly handled with his arm up. El Kaabi stepped up to complete his hat-trick, calmly slotting the ball past Olsen.
It was another moment for Villa to reflect on Martinez, after his penalty heroics in the quarter-final second leg in Lille. Olympiacos added a fortuitous fourth when midfielder Santiago Hezze’s shot took a wicked deflection to leave Olsen wrong-footed.
Villa then wasted a glorious chance to score a third goal when Douglas Luiz placed his penalty wide, after substitute Jhon Duran was fouled.
While Emery is around, anything is possible yet his hopes of reaching yet another European final are hanging in the balance.
Match details
Aston Villa (4-4-2): Olsen; Cash, Konsa, Lenglet, Digne; Rogers (Duran, 74), McGinn, Douglas Luiz, Diaby (Iroegbunam, 85); Bailey (Zaniolo, 85), Watkins.
Substitutes not used: Gauci (g), Wright (g), Carlos, Torres, Kesler-Hayden, Munroe, Kellyman, Young.
Booked: Diaby.
Goals: Watkins (45), Diaby (52).
Olympiacos (4-3-2-1): Tzolakis; Rodinei, Retsos, Carmo, Ortega (Richards, 46); Hezze, Iborra, Chiquinho; Podence (Horta, 70), Fortounis; El Kaabi.
Substitutes not used: Paschalakis (g), Papadoudis (g), El-Arabi, Alexandropoulos, Quini, Carvalho, Jovetic, Prekates.
Booked: Rodinei.
Goals: El Kaabi (16, 29, 56 pen), Hezze (67).
Referee: Marco Guida (Italy).
