Inter striker Romelu Lukaku "doesn't have that little bit more against the best teams", according to Paolo Di Canio, who has poured scorn over his impressive record since joining the club.
Lukaku completed a £74 million ($96m) move to San Siro from Manchester United on August 7, having largely failed to live up to expectations over the course of his two years at Old Trafford.
The 26-year-old has made the transition from the Premier League to Serie A seamlessly, scoring 14 goals in 19 appearances to help the Nerazzurri rise to second in the standings.
Lukaku failed to find the net in his latest outing, but he did provide the assist for Lautaro Martinez to score as Inter were held to a 1-1 draw by fourth-placed Atalanta at home on Saturday.
Di Canio, who enjoyed spells at Lazio, Juventus, AC Milan and Napoli during his playing days, says he has followed Lukaku from his early days in the Premier League at Chelsea to now, but has yet to see him raise his game when it matters most.
"I have always said that he will score many goals and that he will be more and more important for Inter, because for eight years I have followed him - since 2011, when he arrived in England," Di Canio told the Sky Calcio Club programme on Sky Sport Italia .
"I wanted him before [Antonio] Conte, I have followed him since Chelsea and West Bromwich Albion. But my feeling is that against [teams like] Genoa he scores a wonderful goal, but against the best teams, he doesn't have that little bit more."
Lukaku did score a brace in Inter's 3-1 win over Napoli at Stadio San Paolo on January 6, but Di Canio was quick to point out that his former club are no longer a strong defensive outfit, having slipped to 11th in the Serie A table.
“It is no surprise to me that Lukaku scores four braces, but against the teams from 10th place down," he added. "So how do I explain the brace against Napoli? Now, do you consider Napoli a great team?
"The club is huge, the stadium is wonderful, with a huge fan base but how does Napoli play at the moment?
"Perhaps worse than SPAL on a defensive level, they are two goals given away."
Inter will then turn their attention back to Serie A, with a trip to Lecce on the cards as they aim to close the two-point gap between themselves and leaders Juve.