AC Milan fell 10 points behind city rivals Inter this afternoon after another uninspiring display at Bologna.
With Inter cruising past Cagliari 3-0 at the San Siro, a win for Leonardo's men was key to their hopes of keeping the pressure on their great rivals in the race for the Scudetto.
But a failure to find the back of the net - although they came mighty close, twice hitting the woodwork - maintained their poor recent run.
Having manoeuvred themselves into second place in Serie A behind Inter, things started to go wrong for the Rossoneri when they were outplayed in the Milan derby.
Since then they have crashed out of the Coppa Italia at the hands of Udinese and were held to a draw by relegation-threatened Livorno in a half-empty San Siro.
And there were signs of unrest in the Milan camp this week when Brazil star Ronaldinho was forced to deny he had held a three-night party in the lead-up to the Inter game.
On top of that, owner Silvio Berlusconi expressed his surprise at the signing of Mancini on loan from the Nerazzurri, saying he disagreed with the move.
It was hardly ideal preparation for a difficult clash against a side battling to preserve their top flight status.
And, although they started well - Andrea Pirlo hammering in a fifth-minute free-kick which produced a fine save from home goalkeeper Roberto Colombo - the hosts will argue they deserved a share of the spoils for their defensive resistance.
Bologna offered little going forward with their only first-half effort falling to former Italy striker Marco Di Vaio but his effort was comfortable for Dida.
Milan dominated possession but could find no way through a stubborn Bologna rearguard. The crowded penalty area forced Milan to try their luck from long range with Pirlo thrashing another shot over the bar with six minutes to go in what was a turgid first half.
Bologna had the first chance after the break, Di Vaio dispossessing Pirlo and setting off on a mazy run towards goal which was halted by a fine challenge from Luca Antonini.
Milan almost broke the deadlock after 57 minutes but luck came to the home side's rescue.
Bologna failed to clear their lines from a corner and the loose ball was turned towards goal by Ronaldinho but his shot cannoned back off the bar.
Colombo then saved well from Marco Borriello as Milan upped the tempo.
The Bologna keeper excelled himself once more with 21 minutes to go, diving bravely at the feet of substitute Klaas Jan Huntelaar after the Holland striker had been played in by Pirlo.
Milan dominated the closing stages with some wayward finishing and a lack of luck hampering their efforts to snatch all three points.
Leonardo's side thought they had won an 84th-minute penalty when Salvatore Lanna appeared to handle Ignazio Abate's right-wing cross but the referee waved play on.
Then, with a minute left, substitute David Beckham played a one-two with Borriello but the former Manchester United and Real Madrid star could only fire his shot wide from the edge of the area.
Milan came agonisingly close again in injury time when Mancini wriggled clear and whipped in a cross for Massimo Ambrosini but his header struck the bar.