"I don't know," Pettine said when asked if Manziel was in Vegas, "I wasn't."
If it turns out Manziel was in Vegas, Pettine said he would find it 'disappointing.'
The embattled Browns quarterback has been warned by the Browns about his "off-field issues" and was briefly demoted to third-string quarterback earlier this season after another party video surfaced on social media.
Manziel has had considerable drama in the spotlight, including a stint in rehab, social media posts depicting him partying and an encounter with police during which he told them he and his girlfriend were fighting in a car in Avon.
Since Manziel was placed in concussion protocol Tuesday, he was not required to attend the game against the Steelers Sunday. However, players in concussion protocol are expected to report to the team facility in Berea at game time.
"It is my understanding that he was not there," owner Jimmy Haslam said Sunday night.
Manziel addressed the rumors only on social media, posting a picture with his dog Saturday evening with the hashtag "#Saturday nights" and the location tag "Avon, Ohio."