Former goalkeeper Day has returned to the club where he spent eight years and made over 250 appearances as a player from 1985 to 1993.
Williams said: "I've known Mervyn a long, long time. He's an ex-Leeds player coming back to the club to do a job.
"He's experienced, he's been a first-team coach, he's worked with Alan Curbishley, he's been at Everton as well and he knows and understands what a manager needs for first-team football.
"Mervyn brings plenty of experience and will do a good job here. We have to have an understanding of how we're going to do things and if we can find some diamonds great."
Source: PA
Source: PA