United pushed hard to sign Grealish, but the Englishman eventually signed an extension with Aston Villa.
Grealish eventually made a £100m to Manchester City last summer.
Asked about United's efforts to do a deal, Grealish told The Telegraph: "I was really close to going, but nothing happened in the end.
"We played United in a pre-season game on Saturday and I wasn't supposed to play in a cup game at Burton. But after the United game I said to [Christian] Purslow and my agent 'if I'm not leaving, I'll sign my new contract'.
"I went in with my dog on Tuesday morning, signed the deal, and played later that day. We all agreed on the clause and if any team hit that, it's a win-win because it means I've had an unbelievable season and Villa got £100 million."
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