"I had a meeting with Alex at Old Trafford and he felt he had got a little gem in Chris.
"He said he was someone with a few rough edges that needed to be smoothed off. I am sure Manchester United have the knowledge and expertise to polish him as a player."
Smalling arrived at Maidstone after leaving Millwall's academy and quickly began to realise his potential.
Former Portsmouth and West Bromwich Albion goalkeeper Williams told the Manchester Evening News: "He needed the experience of playing at a different standard. He did that and has now gone through this rapid movement along the pathway.
"I have been in the game since I was 15 and seen many players come through. If I am being honest I would have liked Chris to have played 40-50 league games for Fulham before making a move.
"I feel that would have done him a power of good. He has not played enough Premier League football and United will have to be patient with him."