He has a ruptured Achilles tendon and will be out for five months.
Dallan is 26 and has played 39 times for Italy. Recently he was a Treviso hero in their Heineken Cup Round 2 victory over Bourgoin. The Italian side stunned their French hosts with a 34-0 victory at Stade Pierre Rajon.
Treviso are currently in second place in Pool 2. They entertain Bath Rugby in Round 3 on December 4 (kick-off 16:00) before the return leg at Bath's Recreation Ground on December 11 (kick-off 14:15)