The Napoli midfielder turned 28 on the same day his side hosted Lazio in Serie A.
Antonio Conte started McTominay for the 11th consecutive league match following his summer arrival from Manchester United.
However, he could not inspire the hosts to a victory as they suffered their third Serie A defeat of the season.
Gustav Isaksen's second half strike secured all three points for Lazio and their second win over Napoli in four days.
McTominay played 82 minutes before being replaced by Giacomo Raspadori as his side chased an equaliser.
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Meanwhile, fellow ex-United man Romelu Lukaku also started, with Scotland team-mate Billy Gilmour coming off the bench.
Sunday's loss meant Napoli missed the chance to return to the top of the table.
Instead, they remain two points behind leaders Atalanta after 15 matches.
It was also their second loss in a week to Lazio following their Coppa Italia meeting on Thursday.
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Tijjani Noslin's hat-trick at the Stadio Olimpico inspired Lazio to a 3-1 win as they booked their place in the quarter-finals.
McTominay was once again involved but only as a second half substitute, playing the final 18 minutes off the bench.
Following Napoli's latest defeat, the Scot has now made 14 appearances across all competitions.
Those matches have seen McTominay score four goals and provide two assists since his arrival.
He will hope for a response this weekend as Napoli travel to Udinese.
"I think this match clearly showed that we are on the right track, because our idea of football is to be aggressive, take the initiative and try to create dangerous situations when in possession," Conte told DAZN after the game.
"We certainly need to improve on our crosses and final ball, this is not the first time we've spent a lot of time in the opposition final third and didn't make more of that.
"I am definitely not disappointed with the performance, because the lads gave their all, showed spirit, determination and were up against a very good Lazio side. They are not up there as a fluke in Serie A and Europe," continued Conte.
"I am not disappointed or unhappy. I know we are on the right track and inevitably there will be some bumps in that road, there will be again in future, but I saw a team that is aggressive, wants to control the game and what we need is more quality in the final third."
Scott McTominay shares heartwarming moment with young Manchester United fan who gives him parting gift as he leaves Old Trafford for final time ahead of joining Napoli