The incident came inside six minutes. Mateta challenged for a high ball just outside the area when Millwall goalkeeper Liam Roberts came out of his box, catching the Palace striker’s head with his high boot.
After a check of the pitchside monitor, referee Michael Oliver sent Roberts off, with Mateta receiving almost 10 minutes of medical attention on the field before being taken off. He was taken straight to hospital.
The Eagles began to rack up the chances with the additional player, and made the breakthrough in the 33rd minute – although not from one of their own. Millwall defender Japhet Tanganga nodded home from Will Hughes’ cross, beating substitute goalkeeper Lukas Jensen.
Seven minutes later, Crystal Palace doubled their lead and Tanganga had another hand. Daniel Munoz turned home from inside the six yard box, but the goal was initially ruled out for offside.
This was overturned by Oliver, and VAR confirmed the goal would stand after the ball came off Tanganga before Munoz turned home – meaning he was not offside.
Millwall gave themselves a lifeline. Matt Turner initially pushed away a cross, but it landed to the incoming Wes Harding, who fired home.
And the Lions began the second half well, and will feel that they should have equalised. But once Crystal Palace settled, it was clear they would see the game out, and added the safety of a third goal in the 81st minute.
It was a deserved goal for Eddie Nketiah, who replaced Mateta. Eberechi Eze’s free kick was nodded on by Chris Richards, before the former Arsenal striker looped a header home to secure Crystal Palace’s spot in Sunday’s draw.