The Egypt international ended his four-year stay with the DW Stadium outfit and will now continue his progress with the Boro
The midfielder has been with the Latics since January 2016 when he joined from Chesterfield and has been the captain of the club since 2017.
Morsy featured in 150 games during his four-year stay with the DW Stadium outfit and last season, despite featuring consistently for the side, the club was relegated to League One following a 12-point deduction handed to them by the FA after they went into administration.
On Friday, Morsy sealed a three-year deal with Middlesbrough after successful medicals and has sent a heartfelt message to Wigan supporters.
“To the Wigan Athletic supporters, I wanted to take this opportunity to write to you to explain my decision to leave the club after five seasons at the DW Stadium,” Morsy wrote on Twitter.
“Many of those years have been as your captain – something I regard as one of the proudest achievements of my career – so I believe I owe you this explanation, especially given how you have always supported me and made me feel like a massive part of this family.
“The events of the last two months have been well publicised. They hurt me in an immeasurable way and I have felt all of the pain you have. We were building something really good here. I was so confident that we were going to bring success to Wigan Athletic as a Championship side this season.
“I had so much belief in this group of players and staff, so much so that even despite the 12-point deduction I felt we were going to be able to keep the club in the Championship. Through the finest of margins, we just missed out and, like you, the players hurt so much because of that.
“I know how tough it’s been for you watching the manager and so many players leave the club in the last few weeks because I have felt exactly the same. All of the players who have left and many of the players that are still with the club deserve to be playing in the Championship or Premier League and that’s something we’ve all had to respect as we’ve seen it unfold.
“On a personal note, I have naturally had to consider my future- for my family, for my international career and for my own personal desire and ambition to play at the highest level.
“These are challenging times and although it has been a long time since the team and supporters have been at the stadium together, I have felt and seen your hurt through social media and the messages I have received. It hurts me that this news will provide more sadness to many of you in this difficult period, though I hope you can understand my reasons.
“You are the club’s biggest strength and I know you will keep fighting to ensure your club enjoys better times soon. Thank you for everything.”
Morsy, who was not involved in Middlesbrough’s opening game of the season against Watford, could play a part when Boro face Barnsley in a League Cup tie on Tuesday.