Arsenal were leading 1-0 at the Emirates midway through the second half when the incident happened.
Lacazette ran into the box and was brought down by Fred.
The United midfielder walked away from the incident – seemingly believing that Lacazette had just lost his footing after knocking the ball forward.
But referee Jonathan Moss gave the foul and the penalty.
Pierre-Emerick Aubameyang then stepped up to covert the spot kick.
But should the penalty have been given?
A fresh angle from the TV cameras appears to show minimal contact between the United and Arsenal players.
More to follow…