Deeney said he, along with his team-mates, were confused when the stadium announcer called out Heurelho Gomes' name as the visitors starting goalkeeper.
They had been training with Pantilimon the previous day, after Hornet's manager Quique Sánchez Flores had informed the squad that the Romanian would be starting the crunch knock out game.
The Watford striker, 27, explained how the team re-entered the changing room before kick-off only to find out there had been a big mis-understanding.
Flores began to instruct the 6 feet 8 inch Pantilimon on how he should distribute the ball. Deeney claims this was 50 seconds before the sides were due to go out.
The Watford team's bemusement was short-lived, however, as they questioned the management on who was actually starting and Flores confirmed again that it was in fact Pantilimon
It would have been a bitter pill to swallow for the ex-Manchester City man. He represented his former club in every game on their run to the FA Cup final in 2013, before being dropped for the final itself by Roberto Mancini and replaced by Joe Hart.