The former football director resigned from his position at the end of last season.
There have been claims Mitchell's departure was a win for Howe in a long-running power battle over transfers, something which he denies.
The Toon manager told ChronicleLive: "It was a complete surprise to me and a big disappointment because we had stability.
"We were planning the summer ahead and that's a big upheaval internally, which we had been through before with Dan Ashworth leaving. I've had that experience before and know the cost that departure comes with."
On a difficult summer market, Howe also said: "Trust me the work that we have put into loads of things that have had no benefit has been there all through the summer.
"But we still have to do the right things and go through the right processes, and we have to go for the best players that we can.
"We have hit roadblocks on a number of occasions through no fault of our own. I've been through that many times in my career as a manager and I look at it very calmly when it happens because the players that you sign have to want to come. Ultimately, if they don't, they're not the right players for us."