Andrews stepped in at centre-half after Ryan Nelsen was forced off the field with injury.
Allardyce admitted: "Keith Andrews dropping in there was a big decision for us.
"Keith dropped in anyway while Ryan came off and then (goalkeeping coach) Bobby Mimms said 'he's played a couple of games for Wolves in central defence'.
"We wanted to keep Steven Nzonzi in the front end of the defence to try and win the balls as they were played up to the front two and (Andrews) and Gael did ever so well against the threat of (Johan) Elmander and (Kevin) Davies.
"Having had the loss of Samba and then Ryan I was a bit worried and concerned but they did a tremendous job - another clean sheet.
"We've had eight clean sheets in 17 games now and that's the basis of why we're sitting in a mid-table position, because the goals have been a struggle for most of the season - but when you get that clean sheet it means when you get one goal you win three points."