The 20-year old Japanese winger scored from the penalty spot as the Gunners defeated Wenger's former side Nagoya Grampus 3-1 on their pre-season tour of Asia and the Frenchman spoke highly of him after the match, suggesting he would not be sent to Dutch club Feyenoord for a second loan.
"It was not planned. When Ryo scored I thought it was a good decision from Arteta," Wenger explained to the club's official website after regular penalty taker Mikel Arteta stepped aside for Miyaichi.
"Ryo was not completely fit to play but it was a special occasion that I did not want him to miss. He just needs to come back to full fitness and then the rest will come along.
"I like his pace, his mobility and his movement off the ball. I like players who can move when others have the ball. There is also his attitude, commitment and desire. That is why I decided to keep him."