Brown played 90 minutes in both of the Gills' opening pre-season fixtures while McCready featured for 45 minutes against Folkestone, but Stimson revealed he is looking for more quality in defence.
He told the club's official website: "They came in and both gave it their best shot but unfortunately we weren't able to offer them anything. We've got a couple of options - we might bring a trialist in, we might go for a transfer or bring someone in on loan.
"Whoever comes in is going to want to play or at least get an opportunity so we need to make sure that they are the right person."
The club will now continue their search for a new centre-back but the performances of youngster Tom Wynter in recent pre-season outings has left Stimson with plenty of food for thought.
He added: "Tom has had a full game and half a game and he's done well. If we're not able to bring someone in then I haven't got a problem with putting Tom in there."