The South London Press has been told that the Eagles were ready to push through a deal for the 30-year-old attacker, who has been a free agent since his contract with the South Londoners ended in June.
Palace wanted Sako to prove his fitness after being ruled out since early February with a fractured ankle. If Puncheon had joined Boro it would have opened up a slot in their 25-man Premier League squad.
The Malian international, right, is being linked with the Teessiders along with Forest and Aston Villa.
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