The 30-year-old England international has been in inspired form so far this term, but his team have won just once in 12 league games and have conceded 22 goals in the process.
Parker commented:”The whole team needs to be more defensive-minded.
“When we get into a winning position we need to see the game out and we are struggling to do that.
“It’s one thing to concede from a 40-yard screamer or when a player beats four defenders and curls one in to the top corner.
“But when an opponent gets a free header three yards from goal, like West Brom’s equaliser on Wednesday night, that is just not acceptable.
“We can’t keep on allowing sloppy goals like these if we are going to turn draws into wins.”