Union, the team against whom the 34-year-old Riquelme made his Boca debut 17 years ago, ended a run of 26 Argentine league matches without tasting victory in the most unlikely of places.
Riquelme made his highly anticipated comeback after taking a seven-month break from football following their defeat in the 2012 Libertadores Cup final.
The former Argentina playmaker captained a Boca side let down by defensive blunders as coach Carlos Bianchi rested several first choice players for Thursday's Libertadores Group One clash at home to Uruguayan champions Nacional.
"We needed this because we've been working hard and the team have been playing well without getting the breaks," said Union's coach Facundo Sava.
"We're going to fight [to avoid relegation] until the numbers say otherwise. We played a great game from the first minute," he told broadcasters Futbol Para Todos on the pitch.
Boca are in the bottom half of the 'Final' championship table with four points after four matches.
'NO CLASS'
Their fans' sole consolation will have been bitter rivals River Plate's 2-0 defeat away to San Lorenzo.
River's coach Ramon Diaz, the last man to steer San Lorenzo to a league title in 2007, was given a good reception by the Saints' faithful but not by referee German Delfino, who immediately sent him to the stands because his team came out late for kick-off.
"What the referee did was lamentable... we arrived late because the police brought us a roundabout way. I asked [Delfino] to delay the start... [referees] with no class shouldn't officiate," an angry Diaz told reporters.
