After Paul Pogba gave the ball away in midfield, Smalling tried to bail him out but his challenge on Richarlison was woefully mistimed.
There is unlikely to be as much of a stonewall penalty all season, as the England centre-back took the Brazil international out with a challenge that was nowhere near the ball.
Sigurdsson stepped up and dispatched the resulting penalty into the top right corner, with David De Gea diving in the opposite direction.
The goal made it a nervous finish to the match for United, who had dominated up to that point.