The 31-year-old midfielder-turned-centre-back has been forced to sit out the start of pre-season training with a groin injury which he suffered while out on a training run.
Stimson saw striker Mark McCammon's season decimated last year after he suffered a hamstring injury early in training and the Gills boss concedes it is a concern.
"If you miss too much in pre-season it can be hard to catch up," he said. "Mark trained on his own and picked up an injury.
"It's about what you do and where you do it. There were a good 10 players back training with Danny Ellis (earlier in the week) and none of them are injured.
"Some of the players have decided to train on their own and you can't argue with them for doing it."