The Nerazzurri were beaten 1-0 at home by Fiorentina on Saturday for their third straight loss in Serie A.
Giacomo Bonaventura headed in a rebound early in the second half after Inter centre forward Romelu Lukaku inexplicably missed a wide-open close-range chance.
It was Fiorentina’s eighth straight win across all competitions, and the Viola extended their unbeaten run to 10 games.
Fiorentina, who face Lech Poznan in the Europa Conference League quarter-finals, had gone 11 straight matches without beating Inter, with their previous victory over the Nerazzurri coming nearly six years ago.
Inter missed out on a chance to move into second behind runaway leaders Napoli, but remained third, two points behind Lazio. Fiorentina moved up to eighth.
“It’s very disappointing,” Inter coach Simone Inzaghi said. “We need to work more, starting with me.”
AAP