The 31-year old suffered a broken thumb at training on Monday and will now be sidelined for up to two months.
"Alan damaged his thumb in training on Monday,” Wright told the club’s official website.
"At this stage we don't think he will need an operation, but he will see a specialist later in the week and we'll have more information after that.
"However, we are looking at six to eight weeks absence as a minimum."
Mannus, who has featured 19 times for the Saints this season, will be replaced by 41-year old player-coach Steve Banks.
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