Reilly suffered the injury during St Mirren Under-20s' 3-2 victory over Celtic on Tuesday night.
Manager Danny Lennon said: "Thomas was injured the other night so that is a major blow for us and the kid."
The 18-year-old has made 14 appearances for Saints and scored a late equaliser in August against Motherwell, who visit St Mirren Park on Friday night in the Clydesdale Bank Premier League.
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