With the arrival of new manager Gordon Strachan at the start of season 2005–06, Wallace found himself on the sidelines for the first few months. However, in late 2005, Strachan began to play Wallace at left-back in place of Mo Camara. This was a strange selection given Wallace's slight build and inexperience of playing in a defensive role. Strachan persisted with this team selection for several weeks to the bemusement of fans who could clearly see Wallace's lack of aptitude for playing in defence. Wallace's own goal in the New Year game against Hearts at Tynecastle (Celtic still won 3–2) highlighted his lack of positional awareness in playing in defence. Towards the end of the season, new signing Mark Wilson replaced him at left back even though it was not his natural position (Wilson is a right back). Wallace played in the 2006 Scottish League Cup Final at left back, due to Wilson being cup-tied.