Udinese inflicted AC Milan's third successive defeat across all competitions after Rodrigo De Paul's second-half strike secured a 2-1 Serie A triumph on Sunday.
Luigi Del Neri's men came into the match in poor shape themselves having lost each of their last three league games, but Milan were unable to capitalise on their second-half superiority, as Udinese joined Napoli and Juventus in beating Vincenzo Montella's charges in just over a week.
Deulofeu was one of Milan's chief threats in the second half, with his trickery and charging runs causing Udinese plenty of issues, but his final pass was often lacking.
And Udinese took full advantage of Milan's wastefulness, with De Paul's 25-yard strike finding the net and adding to the Rossoneri's woes.
Suso, Milan's star man this season, was unsurprisingly crucial as he picked Bonaventura out with a pinpoint cross and the Italy international smashed home from close range.
Udinese responded well, gradually wresting control of the midfield, and they should have levelled in the 24th minute, but Davide Faraoni did not attack Jakub Jankto's cross with enough conviction at the back post and he agonisingly missed the target while colliding with the goal.
The changes did little to upset Udinese's rhythm, however, and they deservedly drew level just after the half-hour mark.
Deulofeu then picked out a clever reverse pass to Carlos Bacca after another darting run, with the Colombian's 20-yard effort fumbled by Orestis Karnezis.
Milan's inability to capitalise on their pressure ultimately contributed to their downfall.
Udinese hit Milan on the break with 17 minutes to go and De Paul beat Donnarumma from 25 yards with a low shot into the bottom left-hand corner, despite the goalkeeper getting a hand to the Argentinian's effort.
