The Gunners signed Saliba from St. Etienne last summer, but the 18-year-old defender returned to the Ligue 1 on loan.
"He is ready," Puel told The Athletic. "He is a fantastic player. He is a young player but with a lot of maturity in his play. Also, in his life. He is a player with a good feeling and spirit.
"It will be hard for us without him but I think it is a fantastic opportunity for Arsenal.
"I think they need a good centre-back and have done for some time, and I think William is the right player.
"I think he can play all the time in the first team at Arsenal."