Steve Tilson bemoaned his Lincoln side's defensive frailties after the 2-1 defeat at Oxford left them in danger of the drop.
Cian Hughton headed the Imps into an early lead in Saturday's League Two clash at the Kassam Stadium, but Asa Hall levelled for the hosts.
Tom Craddock fired in the winner after 76 minutes before Lincoln's Daniel Hone was dismissed for his second bookable offence.
Tilson said: "We're conceding goals too easily. We've got to be better. It was a poor second goal to concede. But I've said to the players, no-one can fault them for their effort.
"We gave ourselves a great start by going in front early on. We had said how important it was to get the first goal. Then we get it, but still don't go on to win.
"We've got to learn to play for 90 minutes. We've got to be positive in training this week and positive to get the result we need next weekend.
"So we have got to dig deep to make sure we are in the League. We've got to be confident and believe we can do it. If we don't have that belief, we haven't got a chance."