Stimson found himself with a selection headache ahead of the match with starting centre halves Garry Richards and Danny Cullip both ruled out, and opted to bring Cox back into the side ahead of on-loan Adam Bygrave.
Talking to the official Gillingham website, Stimson explained: "We had the three centre-halves in Simon King, Danny Cullip and Garry Richards playing well in recent weeks and we have looked solid. Two of those were missing against Hartlepool and when you go to places like that, you need men to stand up and be counted with strong characters. Those two players that were missing give you that. We had to deal with it and get on with it.
"It was a toss-up between young Adam Bygrave and Ian Cox; I thought I'd go for the experience and maybe Cox would handle the conditions better. I got that decision wrong and it has cost us."
Talking about the side's defensive failings against Hartlepool, Stimson said: "It is now time to do the basics and do what you are good at. If you are paid to stop the ball going into our net, you stop it. I just think that one or two players in that area of the pitch didn't do well enough. Every time Hartlepool had a set-piece, they got a clean header on the ball. We put the markers up on the board before the game and certain players didn't do their job, which is surprising because they are the ones with experience. When it is experienced players, it's even more amazing."