Reilly scored in the 3-2 win but had to come off in the second half after sustaining an injury to his ankle. It's now been confirmed it's a break and he'll be out for more than two months while he recovers.
It's a bitter blow as the teenager has impressed on his first team appearances so far and had been due to join a Scotland under-19 training camp in Turkey in January.
Typically, the news comes at a time when one of our other forwards is just getting himself fit again, with Paul McGowan expected to be involved in the first team before the winter break.
Get well soon Thomas!
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Source: Mirren MAD