Juventus ended their poor run with a 3-0 home win over Napoli in the Italian Cup last 16 on Wednesday but Diego's opener caused huge controversy.
Juve coach Ciro Ferrara had presided over four defeats in five games but looks safe in his job for now after a victory sealed by two second-half goals from the little-used Alessandro Del Piero, the second a penalty.
The first goal was the main talking point, however, with Brazilian playmaker Diego, a major flop since his big close-season move from Werder Bremen, netted on 24 minutes despite Napoli's Jesus Datolo lying injured on the floor.
The in-form visitors furiously protested but the strike stood.
Juve face a quarter-final this month against Serie A leaders Inter Milan, who progressed in December with a 1-0 win over Livorno.
Mathieu Flamini earlier spared AC Milan's blushes by netting the winner nine minutes from time in a 2-1 last-16 victory over third tier Novara.
Striker Filippo Inzaghi, now 36 and often a substitute under manager Leonardo this season, had reminded Milan of his worth with a 12th-minute opener and other fine touches.
A second-string Milan side allowed Pablo Gonzalez to score just after the break but the hosts prevailed when Flamini's powerful strike set up a meeting with Udinese or Lumezzane, who play on Thursday.
Catania's Gianvinto Plasmati scored twice early on in a 2-1 win at nine-man Genoa and AS Roma beat second division Triestina 3-1 on Tuesday.
Holders Lazio are at home to Palermo in the last 16 on Thursday.