Simpson told BBC Radio Manchester: "We're just hoping that he can lift his fitness levels.
"I don't think he's fit enough for the 90 minutes he needs to shed a few more pounds but only he can do that."
The former QPR and Leicester City forward joined Macclesfield in January 2010 on an eighteen month contract after impressing during a loan spell at the Moss Rose.
Sappleton has made just two substitute appearances since returning to full training, scoring in the away match at Oxford.
"We can't monitor what he puts in his mouth unfortunately and when he's away from here, we can only monitor when we see him on the scales," Simpson added.
"He's been warned and dealt with internally but basically he's got to lose the pounds to give himself a chance to play football for 90 minutes.
"He's heavier now than when he got the injury so at the end of the day that should say something to him about looking after his body so he doesn't get that injury again.
"He's the best finisher at the club without a shadow of a doubt and we would love to have him in this team, but we've got to have him effective so he can work for us because he couldn't get through the work at the minute that Tyrone's (Barnett) getting through."
Source: BBC Sport
Source: BBC Sport