Inter's winless run was stretched to four matches in all competitions as Sinisa Mihajlovic's struggling Bologna triumphed 1-0 at San Siro, in the process securing their first Serie A win since September.
With the distraction of the transfer window out of the way, Ivan Perisic was back for the Nerazzurri but they still lacked fluency as they followed up a Coppa Italia exit on penalties to Lazio by losing at home.
Bologna - led by ex-Inter man Mihajlovic in the first match of his second stint as coach - fully deserved a half-time lead secured by Federico Santander, who threatened throughout.
And Inter's response after the break was tame, with Luciano Spalletti's team struggling to service Mauro Icardi as they made it far too easy for their determined opponents.
Icardi should have scored inside the first minute, blazing wide after a dreadful Andrea Poli back pass, but Bologna soon came into their own.
Samir Handanovic was alert to parry from Riccardo Orsolini and then had to make a wonderful stop to deny the dangerous Santander.
Inter were on the ropes and they fell behind when Santander flicked a near-post header into the net from a left-wing corner for Bologna, with a dismal first half capped as Matias Vecino hacked at a clear opportunity at the other end.
Lukasz Skorupski blocked bravely from Radja Nainggolan shortly after the restart, before a long throw presented an opportunity for half-time substitute Lautaro Martinez, who wastefully screwed his header wide.
Spalletti sent on Andrea Ranocchia to add size to the attack with 15 minutes remaining as Inter pushed for an equaliser, but Skorupski beat away a left-footed volley from the centre-back and Bologna held on.
What does it mean? Spalletti's stars slumping
Inter have struggled for consistency under Spalletti, but this performance was worryingly predictable. After a draw against Sassuolo, defeat at Torino and a shoot-out loss in midweek, the Nerazzurri were poor again. Aimless in attack and open at the back, the return of Perisic - linked with a move to Arsenal in January - did little to aid their cause.
The home side did not direct many efforts on target, but Skorupski was there when they did threaten to score. The goalkeeper also brilliantly challenged the advancing Icardi late in the first half. When Inter delivered cross after cross in the second period, the Poland international was there every time, coming off his line to command the situation.
Stefan de Vrij never got to grips with Santander, twice failing to clear the ball in the build-up to one early opportunity before he was beaten to the corner from which the forward scored the only goal of the contest.
Inter next go to Parma, who stunned Juventus in a 3-3 draw on Saturday. Bologna, meanwhile, welcome Genoa the following day.
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