According to Tuttosport, McTominay has successfully undergone cardiac ablation surgery and is expected to return to training on September 22.

Yesterday morning, Napoli midfielder McTominay underwent cardiac ablation surgery at Manchester Spire Hospital.
The surgery was very successful. He will now rest for around two weeks before returning to training. According to Napoli's plans, McTominay should resume training on September 22, two days after the team's match in Florence.
The club hopes he can return after the long international break and be available for the league match against Frosinone at the Maradona Stadium on October 10. Heart specialists believe this expectation may be optimistic, although recovery techniques for professional athletes after such procedures have improved significantly.
Preparing for Napoli vs Arsenal: Anguissa out, Neres a doubt
McTominay will be unable to play against Arsenal tonight, and Napoli's midfield has also suffered injury problems. The club should have focused its recruitment efforts on midfield this summer because Elmas had already left, but Napoli ultimately took a risk and kept only Anguissa, Lobotka, Gilmour and McTominay. After just three matches, two key midfielders have already been sidelined.
McTominay was suspended after the defeat to Como due to atrial fibrillation, and now Anguissa has also suffered an injury. Allegri confirmed yesterday: "He will not play tomorrow." The Cameroon midfielder injured his flexor while attempting to score from a pass delivered by Lucca.
Neres also did not take part in yesterday's training due to an ongoing knee issue. The Brazilian player was sidelined in January this year because of a tear to the peroneal tendon in his ankle, and the knee problem later resurfaced, delaying his return to full team training. Last Saturday, Allegri brought Neres on for 28 minutes late in Napoli's match against Inter Milan. Previously, he played 10 minutes against Como, but his knee made unusual noises again. Final tests will be conducted this morning, and if Neres still feels pain, he will also miss the Champions League opener against Arsenal.