The 21-year-old academy international has made six appearances for Saints and l ast season played eight games on dual registration with Rochdale.
Saints chief executive Mike Rush said: "It's clear that Dougie needs to be playing week in week out and, although the reserves competition offers him chance to play at a higher level, moving to Leigh on loan will see him play more regularly and consistently.
"Dougie is a hard working player and deserves the opportunity to gain more game time. We'd like to thank the Centurions for agreeing to take him on loan and we wish him all the best for the next month."
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Leigh, who began the year as clear promotion favourites, have hit an early-season injury crisis that has robbed them of the services of Gregg McNally, Bob Beswick and Martyn Ridyard and they were also without Rangi Chase for Sunday's 32-32 draw at Bradford.
Leigh owner Derek Beaumont said: "Lewis comes with big wraps from Tommy Martyn and has obviously been well schooled coming through the ranks with Saints.
"We are depleted at the moment with injuries and Lewis will certainly add to our squad. We are backs against the wall at the moment but we are united in what we are doing and I would like to thank Mike Rush and Keiron Cunningham for their assistance on this."
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