Referee Willie Collum momentarily stopped the match minutes into the second half when the score was 0-0 after the object entered the field of play when Sinclair was going over to take a corner kick.
The official informed the match commander of the incident and the game then continued.
Just a few minutes later, Celtic opened the scoring through James Forrest and a flare was thrown on to the pitch from the South Stand, which houses the away support.
Celtic went on to win the match 2-0.