Brazilian forward Richarlison impressed during his one season at Vicarage Road last term following a switch from Fluminense in August 2017.
It was seen as a real coup for the Hornets when they landed him, and his promising campaign saw Everton pay a figure reported to be in the region of £45 million ($57.4m) to sign the forward ahead of this season.
Still just 21, Richarlison has been even more impressive at Goodison Park, scoring seven times in his 13 Premier League outings, with Marco Silva using him in a more central role in recent weeks.
On Monday he will come up against Watford for the first time since departing, but Deeney holds no grudge about his exit, even if he suggested the Brazilian will receive a bit of a bruising.
"I wasn't disappointed [when Richarlison left] because you can understand it," Deeney told Watford's official website. "Business-wise it was a no-brainer for the money.
"You can see his potential daily, his enthusiasm for football is up there and he's now getting the national team looking at him.
"But I don't want to talk him up too much because I'm going to have to smash him on Monday! On Monday he's the enemy."
Richarlison is not the only man at Everton who will be greeted by some familiar faces on Monday, with manager Silva also set to face his old club.
It was the Portuguese coach that brought Richarlison to Vicarage Road at the start of last season and the pair’s excellent relationship led to the Brazilian’s move to Merseyside.
Altogether, Silva spent less than eight months with the Hornets, guiding them to a hugely impressive start to the 2017-18 campaign, before interest from Everton turned his head and coincided with a drop in form for the Hornets.
A win for the Toffees on Monday will move them back above Manchester United and into sixth, while victory for Watford will put them level with their opponents on 23 points.