Forbes was today forced out of the squad who were to fly to San Diego tonight for the fourth leg of the International Rugby Board series this weekend.
Coach Gordon Tietjens has not yet decided on a replacement captain. The gap in the 12-man squad will be filled by debut Taranaki forward Ben Souness.
Forbes suffered a suspected hairline fracture in his left foot late on the first day at Wellington on Friday.
He played in pain through the three knockout games on Saturday, including the 17-19 loss to England in the final.
The 26-year-old was named in the San Diego squad yesterday but withdrew today.
There are now three injury-enforced changes from the squad in Wellington, with Nigel Hunt and Chad Tuoro having replaced Tomasi Cama and Edwin Cocker.
New Plymouth electrician Souness was named in Tietjens' 24-man training squad following last month's national championship at Queenstown.
The 23-year-old is a former New Zealand under-21 triallist whose goal this year is to play for Taranaki A.
New Zealand Sevens squad: Paul Grant (Otago), Nigel Hunt (Bay of Plenty), Zar Lawrence (Bay of Plenty), Tim Nanai Williams (Counties Manukau), Tim Mikkelson (Waikato), Lote Raikabula (Manawatu), Ben Souness (Taranaki), Save Tokula (Waikato), Nafi Tuitavake (North Harbour), Chad Tuoro (Counties Manukau), Tu Umaga Marshall (Wellington), Viliame Waqaseduadua (North Harbour).