The Teessiders have confirmed the former Celtic midfielder, who is out of contract next month, and Bayern Munich II defender will not remain at the Riverside Stadium as manager Tony Mowbray starts to rebuild his squad on a tight budget, while Andrew Davies is expected to return to Stoke following his loan spell.
Mowbray told the club's official website: "I'm aware that Willo started very well when he came here and I can see exactly what assets he brings to the team.
"But I'm looking to put my own stamp on the team and I need to create space in the squad to enable me to bring in my own
players.
"We did have an option to give Maxi a further year on his contract, but I have decided to let him move on.
"Andrew did very well for us at Millwall, but unfortunately has been injured for much of the time since then, which has limited the number of games he has played for us."
Source: PA
Source: PA