Many fans are still upset at how two young talents were allowed to leave the club.
The Magpies are in contention for a top four position this season, but could have used the additional squad depth.
Mitchell told fans at the We Are United event at the STACK: "A lot of people won't believe I'm 21! I looked a lot better pre-July than I did post-September.
"But no, I think it is a complex landscape in the world of football. Definitely for myself I probably underestimated that coming in.
"I would like to say Darren, Eddie and the team in June, in what was a very difficult situation for the club, allowed us to have a platform where we don't have a points deduction."
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