Mateta was rushed to hospital after a heavy collision with Millwall goalkeeper Liam Roberts, who was sent off for the incident.
Nketiah struck in the 3-1 win and later said, "I'm gutted for JP. He's a big character and I wish him a speedy recovery. I'm happy we got the win for him and go into the next round.
"This is unfortunately football sometimes. I want to play minutes, I work hard every day to do that, and today I got the opportunity - not in the best circumstances.
"I felt good, I knew it was going to be a tough game, had to stay patient and when the moment came, I had to be ready to take it.
"I'm ready, I pride myself on working hard every day. I'm ready and hopefully more opportunities will come, I just wan to be out there and help the team."
On whether Palace can win the FA Cup, he added: "I've been able to experience that with Arsenal, so it would be nice to do it with Palace. We're one step closer and if we do well in next round we'll be at Wembley."