On-loan Notts County midfielder Joss Labadie fired Torquay into a third minute lead with his fourth goal since joining the Gulls but Rory McArdle levelled four minutes later, heading home skipper Gary Jones' free kick.
Garry Thompson put the visitors 2-1 up before Thompson crossed for striker James Hanson to head a third and leave Knill to reflect on a forgettable afternoon at Plainmoor.
"That is probably as poor as we have been since I have been here," he said. "We looked flat from start to finish. All the things you work hard to build up go out the window if you perform like that.
"In our position you cannot afford to take your foot of the pedal and if you don't compete then this is what happens.
"We had a great start but from a great start we were 2-1 down and then we lost our keeper. He took a knock in the ribs at Northampton and another one day here but we are hoping he will be all right for Saturday at Gillingham.
"At half time we said to them the next goal is crucial. If we get it then it's game on but they got it and at the end of the day you have got to say we were beaten by a much better team.
"After each game you try and find positives but it's hard to find any positives after that. We are in this position because of performances like that.
"It's pretty difficult to take any positives when you lose 3-1 at home."