Moyes wrote for the London Evening Standard: "The transfer window may have shut but we added a player to our squad this week — someone of top quality on and off the pitch.
"I wanted 'Pat' Evra for a number of reasons, not least because he has great leadership qualities; he is someone who motivates the dressing room.
"I've known Pat, I've worked with him at Manchester United and I know what he is capable of.
"Yes, he's 36 years old and hasn't played since November but he has been training regularly and hard since he left Marseille — he has been out in Dubai with his fitness coach for the past month so he is in good condition.
"When I was at Manchester United, Pat was a really good pro. The biggest compliment I can pay him is that he is a man, he will stand up, he's honest but he will take responsibility.
"Ideally, he can play in any of the three left-sided positions for us — as a centre- back on the left of a three, as a left-back or as a wing-back as he did for Juventus.With Arthur missing for six games, I felt we had lost a little bit of balance down our left side so, with Pat here, it just adds to our options."