Widnes were too "soft" as they lost 30-22 to 12-man Huddersfield Giants, says Vikings head coach Denis Betts.
Giants forward Brett Ferres was sent off after 12 minutes for punching Rhys Hanbury but the home side were unable to take advantage.
Widnes led 10-6 at the break but conceded four tries in the second half in their third home loss of 2015.
"We're disappointed because we couldn't capitalise on a good start," Betts, 45, told BBC Radio Merseyside.
"Mentally, we just got a little bit soft on ourselves and conceded some pretty soft tries.
"You have to work hard at it and find a way of manipulating the space and we were just unable to capitalise."
The defeat means Widnes sit 10th in Super League, a point off the top eight places, but the two teams above them still have a game in hand.
"We've still got so much to play for this year, even if we fall below the line and going into the Super Eights and compete with Championship clubs," added Betts.
"We're still in the Challenge Cup so we've got to come out fighting next week against St Helens (in the last eight). With the cup, you never know."