Maurkice Pouncey of the Pittsburgh Steelers apologized Monday after photos surfaced of the center and his brother wearing "Free Hernandez" hats.
"I fully recognize the seriousness of the situation involving my former teammate, and I regret that my actions appear to make light of that serious situation. I apologize to anyone who was offended by my actions," he said on Twitter.
His brother, who plays for the Miami Dolphins, has not issued an apology.
The two played with the former New England Patriots tight end Aaron Hernandez when they were at the University of Florida.
The two were with Hernandez at a Boston nightclub the night of a 2007 shooting that left two men wounded, for which Hernandez was questioned.
Hernandez is charged with the murder of semi-professional football player Odin Lloyd, and has pleaded not guilty. He was dropped from the New England Patriots shortly after his arrest, and is held without bail.