Alexis Sanchez set up Cristiano Biraghi to give Inter the lead but then substitute Ashley Young conceded a penalty which Domenico Berardi scored.
Borja Valero and Sassuolo’s Giangiacomo Magnani scored late tap-ins before Inter had Milan Skriniar sent off.
Inter’s Roberto Gagliardini hit the crossbar from close range and former Manchester United and Everton striker Lukaku also had a goal disallowed for offside in an entertaining match.
Six former Premier League players featured for Antonio Conte’s Inter during the game. Their slim title hopes are probably now over, sitting eight points behind leaders Juventus in third.