New Zealander Campbell, who is the American boat's designated medic, caught his finger in a pulley block on the mainsheet, the rope that controls the mainsail on the boat.
"Campbell is now resting in his bunk. He's all right but is suffering a bit of shock, so we are going through to the race office for some medical advice," his father said.
"It happened at about 10.30 at night. The seas were lumpy but there wasn't much wind, so Campbell was pushing the boom across, and as the sail eased out he got his finger caught in the main sheet block.
"Losing the end of a finger must be a family trait - I managed to do the same thing a couple of years ago."
While Ross Field skippered the winning boat in the last Whitbread race, Campbell is a fully qualified skipper in his own right, and in 1995 was in charge of the winning boat in the Swan class World Championships.
While Campbell nursed his injured finger, America's Challenge was lying seventh in the 10-boat Whitbread fleet, now closing on the first-leg finish in Cape Town.
The leader, the Paul Cayard skippered EF Language, is 740 miles ahead of Field's boat, and 113 head of the second-placed Merit Cup. However, Cayard had run into lighter winds and was slowing, while Merit Cup and the third-placed Innovation Kvaerner, skippered by Knut Frostad, were picking up speed.
WHITBREAD ROUND THE WORLD RACE (first leg, 7,350 miles, Southampton to Cape Town) Latest positions: 1 EF Language (Swe) P Cayard 1,195 miles to finish; 2 Merit Cup (Monaco) G Dalton +113 miles; 3 Innovation Kvaerner (Nor) K Frostad +127; 4 Silk Cut (GB) L Smith +416; 5 Chessie Racing (US) M Fischer +627; 6 Toshiba (US) C Dickson +728; 7 America's Challenge (US) R Field +739; 8 Swedish Match (Swe) G Krantz +835; 9 EF Education (Swe) C Guillou +972; 10 Brunel Sunergy (Neth) H Bouscholte +1,065.Reuse content