Borriello, who joined Roma from Milan in August to make room for Zlatan Ibrahimovic, was fortunate to score on 68 minutes when home defender Ignazio Abate kicked a Jeremy Menez cross against his ex-team mate and the ball rolled into the goal.
He was later substituted and boos rang out from Milan fans as he applauded the small cheering Roma contingent, who were witnessing Milan's first Serie A defeat since October 30.
Former Barcelona and Inter Milan front-man Ibrahimovic has been the catalyst for Milan's resurgence this term as they bid for a first scudetto since 2004 but the giant Swede was slightly off the pace after a busy few months spearheading the side.
Midfielder Andrea Pirlo will also need the recovery time after limping off with a thigh injury midway through the first period which led to Milan dropping their intensity slightly against Claudio Ranieri's solid defence.
"It's a great emotion to return to the San Siro," former Inter forward Adriano, who has complained about his lack of match action since arriving from Flamengo in June, told Sky.
"I'm happy but it's been a difficult match."
Milan's defeat meant stuttering champions Inter, who won the World Club Cup on Saturday but are down in seventh in Serie A, stay 13 points adrift of the leaders but with two games in hand.
In Serie A's other Saturday clash, Chile's ex-Inter midfielder Luis Jimenez struck on 17 minutes to give third-bottom Cesena a 1-0 win over Roberto Donadoni's Cagliari.
