The attacking midfielder looked on track for a return to the Sydney side after undergoing a left knee reconstruction late last year, but an arthroscope has revealed cartilage damage in the same knee that could see Enfield sidelined for up to 12 weeks.
He will begin treatment for his injury immediately, with FC planning to bring in a short-term replacement for the 24 year old American.
Enfield joins Brendon Santalab on the wounded list.
Santalab suffered a slight quad strain at training last week and will miss Thursday?s trial against state Premier League powerhouse, Sutherland.
Despite the injuries, Coach John Kosmina is confident his team can build on the good work it did last week against Blacktown City.
?We?d only started practicing football stuff the day before we played the Demons so I was happy with the performance, and if we can build on that this Thursday I think we?ll be in good shape leading up to the Pre-Season Cup.?