The defeat, their ninth for the season, was also compounded by serious injuries to forwards Trevor Thurling and Joel Thompson, which continues the Raiders' misery this year when it comes to long-term casualties.
Dugan will undergo scans later on Monday, but the initial diagnosis suggests damage to an ankle ligament and a stint on the sidelines lasting anywhere from four to eight weeks.
It's the second time the Raiders star has come under an injury cloud this season, having lost a month with shoulder troubles after Canberra were beaten by Sydney Roosters in round three.
The Raiders will also sorely miss Thurling, who could be out of action for six to eight weeks after injuring the AC joint in his shoulder, while Thompson will have an x-ray on Monday on a suspected broken hand.
Meanwhile, Joe Picker and Jarrad Kennedy are back at training after recovering from skin infections - the second time in five years the Raiders have been hit with a boil outbreak.