The Airlie Birds lost 29-18 on Thursday against opponents who were down in ninth spot.
It was a second defeat in three outings for Lee Radford’s side, who started the round in fifth place.
Hull struggled for any strike, made too many unforced errors and were far too lax in defence.
Utility player Washbrook, 32, said: “I thought Giants played really well.
“They defended really well especially; we tried to throw a lot at them and every time it looked like we might be half through a gap or might have the numbers on them, they turned up and forced some errors.
“They either forced us into touch, that kind of thing, or made last-ditch tackles.
“But, yes, we certainly do (need to eradicate such losses); if we’d have won it, it would have kept us in touch with that top-four.
“Obviously we don’t want anyone above us to get too far away from us as it is hard to claw back.
“We’ll have to put that right next week. We’ve got a tough game at home to Saints and we do need to sort it out.”
Hull have made the Super League semi-finals for the last two seasons and have targeted a first Grand Final since 2006.
That is especially so since their Challenge Cup run – FC won at Wembley in 2016 and 2017 – was ended by St Helens in the quarter-finals.
Saints, of course, are the clear Super League leaders and ex-Wakefield Trinity player Washbrook conceded: “They are still the stand-out team.
“They’re at the top of the league for a reason.
“But we ran them close in the Cup and showed we can live with them and put them under some pressure.
“We have to just make sure we’re bang at it next Friday.”