He said he only told team leadership about his plan to leave the tunnel for the national anthem just before the game.
Earlier Monday, quarterback Ben Roethlisberger said the team agreed in a players-only meeting late Saturday night that the team would stay in the tunnel for the anthem as a show of team unity.
Villanueva, a retired Army Ranger and veteran of combat in Afghanistan, said he received numerous texts from other veterans before the game imploring him to salute the anthem. He also noted the game was played at Chicago's Soldier Field, which is named to honor military veterans.
Alejandro Villanueva explains how Sunday's events unfolded & apologizes for making Mike Tomlin & Steelers look bad pic.twitter.com/fl28I3b85L
Villanueva confirmed statements made by Roethlisberger and defensive lineman Cam Heyward earlier in the day that they wanted to stand with him to support him but they were blocked by people going through the tunnel until the anthem began.
He said he had no doubt that if his teammates had known his intentions, they all would have stood with him, even those who were inclined to kneel or sit during the anthem.
Al Villanueva says when he sees the picture of himself standing alone he's "embarrassed" #WPXI#Steelers
"When everyone sees an image of me by myself, everybody thinks the team and the Steelers are not behind me, and that's absolutely wrong," he said. "It's quite the opposite."
Villanueva apologized to his teammates, head coach Mike Tomlin and the franchise, saying he felt "embarrassed" by photos showing him standing alone and appearing to break the image of unity the team wanted to project.