The Gills slumped to a second consecutive defeat on their travels following Saturday's 4-2 thumping at Tranmere.
Stimson said: "We're trying to improve as much as we can but we can only work as part of our financial budget."
Curtis Weston put Gillingham ahead but Scott Vernon and Kevin Lisbie secured three points for the U's.
Stimson added: "We're a decent side but we have to cut out the defensive mistakes and get a win or two.
"Hopefully we will have a few key men back soon. The boys performed so well without them, so it's a little easy to forget that. They have come here and put in a good show but we've ended up getting nothing from the game."