Wolverhampton Wanderers were beaten 3-1 by Middlesbrough at the weekend.
Wolves were 1-0 up against Boro, but they collapsed in the second-half and ultimately ended up losing the contest 3-1.
The defeat means that Wolves have now failed to win any of their last three matches, with their defence looking particularly vulnerable.
And Jackett said: “We need to improve in certain areas, we understand that, and we are very thin on numbers now and ran out of recognised defenders which I felt cost us.
“We have now conceded nine goals in three games and that doesn’t give you a chance to earn points."
Jackett has already stated, in the Express and Star, a desire to bring in more players over the coming weeks, as he looks to address the weak points in his squad.
The Wolves boss has said that he will look to the loan market to bring in further quality, and some new defenders are likely to be his top priority.
Wolves have now conceded 20 goals in their first 13 Championship matches this season, and they have failed to live up to expectations so far this campaign.
Jackett's men were tipped to be among the most likely sides to be promoted this term, but they are currently in 14th place in the table and already nine points adrift of the play-offs.