Curle explains the reasons behind Turnbull's shock exit from Cobblers
He made 42 appearances and scored five goals, but will not be signing a new deal as the club return to League One.
Curle explained: "Jordan wasn't really in the frame when I first came in but since I've been here he's played a vital part of what we've built.
"With the current situation and also the club's situation, we've had to show good housekeeping and there's a financial restructuring that has had to take place.
"Within that, I didn't feel it was right to restructure the finances that would have seen Jordan potentially be on less money for doing the same job.
"If you do that and if you stay in the same environment, I don't think you can perform your best and that's how I explained it to Jordan.
"The restructuring and the housekeeping that we have to show, and the environment that Jordan was going to be working in, I didn't think I would be able to get the best out of him and he wouldn't be able to get the best out of himself.
"So it's a fresh start for Jordan and a new challenge and I think that's what he needs at this time. It's not a money decision, it's a decision by myself that I think is right for me, for the club and also for Jordan."