Pools saw a 2-0 lead evaporate as depleted Stags hit back with three goals in 11 second half minutes.
Gavan Holohan thought he had snatched a stoppage time leveller but, as he celebrated with the army of over 1,000 away fans, a late linesman's flag ruled him offside.
“From my angle I don't see how it was offside,” said Lee.
“He made a run and came across one of their defenders and knocked it in.
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“The linesman believed he was in an offside position before the kick was taken.
“I need to watch it again. If he has got it right then well played, but from my angle it looks a clear goal.
“But we shouldn't be looking for a last minute equaliser when we were 2-0 up. We should see the game out or at least keep doing the right things.”
Lee continued: “It was a tough challenge, we know they're a good team. But when you are 2-0 up I would expect us to win.“I am very disappointed. We knew we were up against a tough team and we probably sat off them too much first half.
“The first time we got hold of the ball we scored. That gave us a bit of belief and it was a great start to the second half to go 2-0 up.
“So it was disappointing to then see us stop what we'd been doing, sit off, and stop pressing them.
“We dealt with crosses really well first half but second half we looked like a different team and no one made the first contacts. It cost us.”