The Brazilian winger joined Arsenal in August 2020 after seven years at Chelsea, where he had established himself as a key player and won major honours. He signed a three-year contract with Mikel Arteta's side after Chelsea offered him only two more years.
Willian said Arteta convinced him to make the move by promising that he would become the central figure in the team.
"I met Arteta, I met everyone there. And they said 'we want you to be the main guy'," Willian said on The Obi One Podcast with former Chelsea team-mate John Obi Mikel.
"Arteta said to me: 'We're going to play like this and I want you to be the main guy, I will build the team around you, I want to give you the key to the club'."
The decision quickly became one Willian regretted. He said he told his agent after three months that he wanted to leave, having become frustrated because what he experienced at Arsenal did not match what he had expected.
"I got frustrated," he said. "I am not saying it is their fault, but when you heard a lot of things and then you join the club, things that are not the same as what they told me, I got frustrated straight away."
Willian eventually left Arsenal by mutual consent after one season, scoring once in 37 appearances before returning to boyhood club Corinthians. He has since described the spell as the worst period of his professional career.
The winger said his unhappiness affected his performances, despite Arsenal giving him the most lucrative contract of his career.
"It was the best contract of my life there, but I wasn't happy," he said. "When you are not happy you cannot perform. Money cannot buy happiness."
Willian also criticised the difference in mentality between Arsenal and Chelsea, where he had been competing for trophies.
"The mentality was totally different to Chelsea," he said. "When I left Chelsea they were fighting for titles every season, but Arsenal were in the middle of the situation where people were leaving, people were coming, and it was a difficult situation."
He also questioned aspects of Arteta's management, while making clear that he respected the Spaniard's coaching ability.
"He's a great coach inside the pitch, his ideas are great but he missed little bits on how to manage the team," Willian said. "The human part, he was missing it."