It could have been worse for Roma, who salvaged a 2-2 draw after appearing to be heading for a loss on Sunday. They trailed by two fine goals from Antonio Candreva before Belotti nabbed a late equaliser.
Roma's city rivals Lazio also dropped points, conceding twice late on to lose 2-1 at Lecce, while Juventus won 3-0 at Udinese and Atalanta beat Sassuolo 2-0.
With star forward Paulo Dybala suspended - along with attacking midfielder Lorenzo Pellegrini and coach Mourinho - Belotti was handed a rare start for Roma.
He thought he had finally netted his first league goal for the club on seven minutes - after a year in Rome - but his strike was ruled offside.
Belotti didn't have to wait too much longer, though, opening the scoring with a shot from the middle of the penalty area 10 minutes later.
Candreva levelled shortly before halftime, collecting a pass from Federico Fazio and turning defender Chris Smalling inside out before firing into the roof of the net.
Candreva's second goal, four minutes after the break, was even more impressive.
The 36-year-old controlled a crossfield pass and twisted his way past Bryan Cristante before curling the ball into the far corner.
The strike appeared to set up Salernitana for their first-ever win over Roma before Belotti headed in a corner with eight minutes remaining.
Juventus's hopes of putting a miserable 2022/23 season behind them got off to a flying start with a 3-0 victory at Udinese.
Federico Chiesa opened their account in the second minute before Dusan Vlahovic and Adrien Rabiot completed the scoring before halftime.
Hardly anything went right for Juventus last season: they were hit with a 10-point penalty over false-accounting and finished a lowly seventh in Serie A. They were also handed a ban from UEFA competitions.
Charles De Ketelaere scored his first goal in Italian football on his debut for Atalanta, who he has joined on loan after a disappointing season at AC Milan following a big-money move from Belgian club Brugge.
The Belgium international headed in the opener seven minutes from time before Nadir Zortea sealed the result in stoppage time.
Lazio appeared to be heading for victory following a strike from Ciro Immobile in the 26th minute but Lecce levelled through Pontus Almqvist with five minutes remaining before Federico Di Francesco scored the winner two minutes later.
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