United signed the Brazil international from Shakhtar Donetsk for £48 million in the summer of 2018 but he struggled to feature on a regular basis with the first-team.
Fred scored one goal in only 17 Premier League appearances that season and said he failed to adapt to life in Manchester at the beginning.
"I believe the first season was very difficult for me," he told the Associated Press. "There was the process of settling down here, but also other personal things that were happening in my life, such as the birth of my son.
"So I believe it was a mix of reasons that made me have an awful first season."
United beat rivals Manchester City to Fred's signature and he said he was proud that managers Jose Mourinho and Pep Guardiola were interested in him.
"It was a contest between two great managers and I could only be delighted with that," he added. "It was very important for me as it would have been for any player."
Since Ole Gunnar Solskjaer replaced Mourinho at Old Trafford in December 2018, Fred has become one of the key players in the team especially after Paul Pogba and Scott McTominay suffered injuries this season.
Fred featured in every game for United as they went on an 11-game unbeaten run in all competitions before the coronavirus pandemic and said he is more settled at the club.
"I'm definitely settled in every aspect now," he said. "I've been enjoying some good performances and the chance of playing a great sequence of games. That was also helpful. I'm delighted with this season.
"I'm now playing well and hopefully I can keep the momentum if the league returns. I just want to give my best for this club, trying to be a key part of the team."