Mendieta, who has not featured for Gareth Southgate's first team this season and who played just seven times during the last campaign, looks certain to leave Boro in the summer after five years at the club.
Boro's reserve team boss Steve Agnew praised the 34-year-old's contribution.
He said: "Whenever he comes into the group he's always professional. He does get a lot of respect from the younger players and deservedly so.
"The one thing that's most pleasing is the way he goes about the game and his appetite, work rate and determination. He's first class and it's a pleasure to have him in the reserves."