But he's sure they can bounce back.
The Swans' 15.15 (105) to 7.9 (51) defeat has thrust them into a sudden-death semi final at ANZ Stadium next Saturday night, against the winner of Sunday's Richmond-Carlton elimination final.
But first there will be a searching review to work out how the Swans were outscored 11.8 to 3.2 in the second half, as a match that was dead even at half-time turned into a 54-point loss.
"I don't know. We're going to have to review it and see where we went wrong," a dejected Jack, who along with Josh Kennedy was the Swans' best, said post-match.
"The first half we thought we were right in it, right in the game.
"The second half probably they lifted, they lifted their pressure and we sort of capitulated a bit."
Jack said the Swans hurt themselves with skill errors and allowed the Hawks too many uncontested marks.
"It was something we knew they were going to do but no one really did anything or put their hand up to try to stop it," he said.
The good news for the Swans is they'll have almost two days of preparation more than their semi-final opponents, who will also have to travel interstate.
"That's the positive to come out of it," Jack said.
"We've earnt this right, we've got a second chance.
"We're going to have to regroup pretty quickly and just get back on it."
He's confident the Swans are still in the premiership hunt despite Friday night's deflating loss.
"Absolutely. The first half we thought we were well in with a chance," he said.
"We thought we were right with them.
"But when you give a team like Hawthorn easy ball and turnovers, they're going to make you pay.
"We got some things wrong. We know we can get it right but we just didn't get it right tonight."
