Milan dropped potentially crucial points in their bid to win Serie A after they were frustrated by Bologna in a 0-0 draw at San Siro.
The Rossoneri went into the international break holding a three-point lead on the back of three straight 1-0 wins, but they could not keep up that winning run on Monday.
Indeed, Bologna – whose coach Sinisa Mihajlovic was not present on the touchline as he is in hospital for leukemia treatment – restricted Stefano Pioli's team to little in the way of clear chances in their boss' 100th league match in charge, to the chagrin of an expectant Milan faithful.
With Napoli and Inter having both won on Sunday, Milan's advantage has now been cut to just one point with seven games remaining.
Rafael Leao had the first opportunity 16 minutes in, though the Portugal forward could not keep his shot down.
Michel Aebischer drilled straight at Mike Maignan following Marko Arnautovic's clever dummy, before Milan's goalkeeper tipped Musa Barrow's strike over after Gary Medel had made a fine block at the other end.
Bologna continued to frustrate their hosts after the restart, with Leao particularly wasteful, first skewing a cross out of play before failing to get a shot off when he had space.
Leao provided a sublime pass for Davide Calabria in the 61st minute, but Medel just managed to divert the full-back's shot wide – a touch the referee failed to notice.
Ante Rebic blazed over as Milan kept up the pressure, with Leao then curling wide from just inside the area.
Zlatan Ibrahimovic was given 20 minutes to try and make a difference, but the veteran headed over from the one chance that came his way as Milan were thwarted.
What does it mean? No milestone celebration for Pioli as pressure mounts
Pioli did not look longed for the Milan job early in his tenure, but the work he has done to turn the Rossoneri into genuine title contenders has been quite superb. Yet, his milestone 100th Serie A game in charge did not end in a victory, but instead a draw that could prove decisive.
Inter battled their way to a 1-0 win over Juventus on Sunday and are just four points back with a game in hand, while Napoli, who like Milan have played 31 times, are only a point behind the leaders. Bologna, meanwhile, did their coach Mihajlovic proud.
Medel leads brilliant Bologna rear-guard
Medel spent time at San Siro with Inter and the experienced, dogged defender was excellent at the heart of Bologna's defence, doing his old side a huge favour in the process.
He made a joint team-high two blocks, including a fantastic one from Diaz in the first half, gained possession seven times and kept things simple on the ball, completing 33 of 36 passes before he had to go off with a head injury following a collision with Ibrahimovic.
Leao found lacking as Milan pay for their profligacy
It has been a fine season for Leao overall, with the 22-year-old scoring eight league goals so far.
His finishing was found totally lacking this time out, however, with only one of his eight attempts hitting the target, while four were blocked. His wastefulness summed Milan up, as the hosts tallied up 33 attempts to no avail.
What's next?
Milan travel to Torino on Sunday, while Bologna host Sampdoria.