Barkley left the Toffees in January 2018 in a cut-price deal, with his contract close to expiring at Goodison Park. After a difficult end to that campaign with the Blues, he's emerged as a brilliant option for Sarri this term and is poised to play a key role against his ex-employers.
Sarri said he was impressed with how Barkley has been able to work his way up the pecking order at Stamford Bridge, per Dominic Fifield of the Guardian.
"At the beginning, he was in trouble," Sarri said. "Then he started to improve in every training, in every match. And so I am surprised [at his ability to adapt] if I think back to the first month, but not if I think of the last two months. I like very much him because I think he is a complete player."
With Jorginho and N'Golo Kante nailing down starting spots in midfield early in the campaign, Barkley and Mateo Kovacic have been jostling for the third position in the central trio.
Sarri added that "from a tactical point of view," Barkley is "a step ahead" of fellow England international Ruben Loftus-Cheek.
Speaking to Sky Sports ahead of the game, the 24-year-old has said he's not expecting the best reception from his former supporters:
It was reported in the summer of 2017 that Barkley had agreed to join Chelsea for £35 million, although he backed out of a move late in the transfer window, per Matt Law of the Daily Telegraph.
A hamstring injury meant he didn't play for Everton in the early part of the campaign before he signed for the Blues in the January transfer window for around £20 million less. The decision left many Toffees supporters unhappy with their academy graduate.
Barkley's decision to move on has been vindicated, as he appears to be making steady progress at Chelsea.
Everton manager Marco Silva said he's a fan of the midfielder too:
The challenge for Barkley will be to maintain the levels he has set with the London club on a consistent basis.
During his time at Everton, he would regularly have purple patches like the one he's been enjoying recently, only to embark on a run of matches wherein he was ineffective. While he can be a force of nature at his best, Barkley has a tendency to lose the ball cheaply and makes poor decisions in the final third when he's off his game.
However, with players like Jorginho and Kante alongside him, not to mention Eden Hazard ahead of him, finding that regularity in his play will most likely be more straightforward.
Sunday will be a big moment for Barkley, and if he starts, it will be intriguing to see how he handles the occasion and the jeers that will come his way from the travelling supporters. Should he excel, it would be another step toward becoming the "complete player" Sarri sees him as.