Graham Westley believes the injury absences of Keith Keane and Scott Laird have taken their toll on Preston.
The full-back duo recently began lengthy spells on the sidelines having suffered knee ligament damage and a broken leg respectively during North End's goalless draw with Notts County.
Despite securing an FA Cup third-round berth with victory over npower League Two leaders Gillingham, Preston have subsequently lost back-to-back League One games against Leyton Orient and Crewe.
"In the three games we have played since the two full-backs Keane and Laird went out, we haven't shown the ingredients as a team that it takes to win matches," Deepdale boss Westley told the Lancashire Evening Post.
"Before that we had got ourselves to a decent level of form and performance, and results had been fairly consistent - we were unbeaten in seven.
"In all honesty, the FA Cup win over Gillingham was an unconvincing performance with us spending a lot of the second half on the back foot when really we should have been better.
"We haven't replaced the full-backs in a way that is getting the team functioning and performing."