The centre-back was left dumbfounded by the decision, which led to a late equaliser
The centre-back, currently in his first season with the Serie A club, was penalised for an incident in injury-time in the box by referee Willie Collum.
Lars Stindl tucked away the spot-kick to cancel out Nicolo Zaniolo’s first half effort in the 95th minute at Stadio Olimpico, leaving Paulo Fonseca’s side with only a slim lead at the top of Group J.
However, replays have shown that Smalling was struck in the face with the ball rather than his hand or arm, leading to widespread furore with VAR not being used in the competition until the final.
“[I’m in] disbelief that this decision went against us,” he wrote, along with a clip of the incident. “We deserved the [three] points, so disappointed.”