Chelsea’s defeat at Leicester on Monday night was their ninth in 16 matches as their poor start to the 2015-16 campaign continued.
Stones’ assured displays for the Toffees have led to him being tipped as a solution for Jose Mourinho’s ailing Premier League champions, but that would leave Everton with a huge hole to fill in the heart of their defense.
"When you are right in the middle of a season, I think players understand that you can’t sell someone if you can’t replace him," Martinez added.
"I think the uncertainty comes in the summer, where everyone speaks about possibilities.
"January is a window to solve problems with players who are not featuring as often as they feel they should, or to bring in players from a lower league to give them an opportunity."