I remeber the second leg of the league cup semi against then-Premier League Leicester in the late 90s (edit:year 2000) only attracting 29K, depite regular sell outs in the league...
Lambert leaving of itself won't bring the crowds back in their droves- for the majority of stay-aways I doubt it was an anti-Lambert stance per se, more fed up of the constant lack of entertainment. Might take some a little convincing to resume parting with the cash and time.