McCammon has only started 24 league games in his two years at Priestfield, scoring just five goals.
"I gave him the carrot by asking him if he can be our main man," Hessenthaler told BBC Radio Kent.
"He's got to prove to me now that he's fit he can stop me from going out and getting another striker."
Hessenthaler continued: "Mark has been out for sometime. He's been at the club and hasn't played a lot of football in the last year, but he's part of this football club and it's down to the lad now."
Source: BBC Sport
Source: BBC Sport