"He has had a slight setback but I think they've got to the bottom of that now," Hessenthaler told BBC Radio Kent.
"He's in hospital at this present time but he will be coming out at the weekend so we're staying positive."
Hessenthaler added: "He's got to take it easy now, and then come back perhaps next season ready to go again.
"It's been a tough time for Darren and all our family over the last few weeks.
"I think he's perhaps trying to come back too soon. He just wants to be back at the football club working again.
"But he's had a heart attack and he needs to recover from it."
Source: BBC Sport
Source: BBC Sport