Radja Nainggolan struck late on against his parent club as 10-man Inter's winless run in Serie A extended with a 1-1 draw at home to Cagliari on Sunday.
The Belgian midfielder was loaned to Cagliari for the season in August and found a way through 12 minutes from time to deny Inter a first win in three league games – their misery compounded in stoppage time as Lautaro Martinez was sent off.
Inter debutant Ashley Young had earlier set up Martinez for what appeared to be the winning goal, but the hosts failed to kill off the match and were pegged back.
And Antonio Conte's men were then reduced to 10 men late on when Martinez was shown a straight red card for dissent, as Inter lost further ground on Juventus in the title race.
Former Cagliari star Nicolo Barella had a headed goal ruled out for offside after 14 minutes but Inter did not have to wait much longer to find a breakthrough.
Following a spell of sustained pressure, Martinez profited from Young's inch-perfect cross to head home.
Martinez was denied a second before half-time by a good Alessio Cragno stop following a mazy run, but Samir Handanovic was also required to keep out Paolo Farago's header.
The visitors more than held their own in terms of possession but the chances fell Inter's way, with Stefano Sensi twice coming close and Martinez volleying wide from close range.
However, Inter's failure to add a second goal proved costly as Nainggolan rifled a long-range shot past Handanovic with the help of a deflection off Alessandro Bastoni's heel.
Lukaku pulled a shot wide as Inter went in search of a winner but those hopes were killed off when Martinez was dismissed, the forward being booked for a challenge on Ragnar Klavan and then shown a straight red for his subsequent protestations, which the coaching staff continued at full-time.
What does it mean? Inter lose further ground
Inter have now drawn their three most recent league games and five of their past seven, opening the door for Juventus to take control of the Scudetto race.
The Nerazzurri are struggling to see games through and could find themselves six points adrift of Juventus if the reigning champions beat Napoli later on Sunday.
Young slots right in
Young joined Inter from Manchester United in a reported £1.5million deal last week and Conte wasted no time bringing him into the side for the suspended Antonio Candreva.
The converted wing-back failed to register an assist in 12 league games for United this term, but it took him just 29 minutes to set up Lautaro with an impressive right-wing cross.
Sensi left sweating
Inter CEO Giuseppe Marotta said ahead of the game he is "optimistic" Christian Eriksen will arrive from Tottenham next week, which could be bad news for Sensi.
He is the midfielder most at risk of dropping out and had a mixed game on Sunday, failing to make the most of his opportunities when getting into some promising positions.
Inter host Fiorentina in the Coppa Italia quarter-finals on Wednesday, while Cagliari have a six-day break before facing Parma in their next Serie A outing.