Roma misery deepens with 4-1 home loss to Napoli
The Giallorossi have now lost three of their last four Serie A matches to risk falling out of contention for the Champions League places.
Roma was also recently eliminated from this season's Champions League by Porto, which resulted in the firing of coach Eusebio Di Francesco and the departure of sporting director Ramon "Monchi" Rodriguez.
Milik had the goal of the day, using splendid backheel control before spinning around to score from close range.
Midfielder Steven Nzonzi had a chance to pull one back for Roma in the second half but he headed onto the crossbar in front of an open net.
Napoli's only concern was an apparent injury to Mertens that meant the Belgium international had to come off in the second half. But Mertens said afterward that it was only a slight knock to his knee.
Napoli is all but assured of finishing second, trailing Juventus by 15 points and 10 points ahead of third-placed Inter Milan.
"We deserved the win and it's too bad we didn't score more goals in the first half, too," Mertens said. "We are continuing to develop as a team."
Roma dropped from fifth to seventh and now trails fourth-placed AC Milan by four points.
It's the first time in nearly five years that Roma has lost two straight Serie A matches.
"It's hard to help the team. The opponents run faster and harder and we're doing everything we can," said Claudio Ranieri, Roma's interim coach. "The players told me they haven't had as much fitness training this season, as they had so many injuries and there was no consistency. They had a packed schedule and couldn't train, just play or recuperate.
"I am trying to get them to train, but it's not easy," Ranieri added. "Then there's (Nicolo) Zaniolo, who is in great shape but had a stomach bug on Friday and told me he wasn't in condition to start today. ... Unfortunately, this is the situation."
The top four finishers qualify for the Champions League.
In Milan, Milinkovic-Savic headed in by the far post following a smart cross from Luis Alberto.
Inter sorely missed striker Mauro Icardi, who was left off the squad following a six-week exile from the club amid protracted contract negotiations.
Parma went ahead early through Gervinho but Mario Pasalic quickly equalized before Zapata's decisive goals gave him 19 for the season, tied with Cristiano Ronaldo and Krzysztof Piatek for second on the league chart behind only Fabio Quagliarella's 21 for Sampdoria.
Francesco Vicari scored early with a header for Spal, while Frosinone was unfortunate to hit both the post and crossbar.