Martinez said Everton chairman Bill Kenwright was left feeling "angry, disappointed and hurt" by United's bid.
"There's been a previous relationship of 11 years with a manager and he had a great relationship with the chairman so you can imagine it is extra-sensitive," said Martinez.
"The way it is, you are going to get clubs making bids. When a club financially needs the money, sometimes you can sell players at the value. But at Everton, believe me, we don't need to sell and we don't want to sell so that makes things very easy."
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