Manager Dario Gradi is concerned Crewe`s defensive frailties could cost them promotion.
While the Alex are the top scorers in League Two with 40 goals in 19 matches, they have conceded 28.
It is a problem Gradi believes he must address during the January transfer window.
"I don't think this team is doing well enough defensively to be sure of going up," said Gradi. "We might scrape into the play-offs if we get a bit of luck but to be sure of getting there we have got to improve.
"We have got a lot of games coming up now and, while we are not shipping as many goals as we were last year, we are still conceding too many.
"I want more aerial threat and I will be trying to do that in the transfer window. I think we know what this team can produce now and I will continue to try to improve what I've got but I have got to add some height."