Speculation continues to surround the futures of key duo David Wheater and Robert Huth, but Wheater has made it known he wants to stay at the Riverside.
Balde could be an attractive proposition for Boro.
The Guinea international is available on a free transfer after leaving Celtic at the end of last season.
The 33-year-old had a trial at Premier League new boys Wolves earlier this summer, but failed to win a move to Molineux.
Balde is thought to be keen on a move to England.