Penrith coach Ivan Cleary conceded the side was guilty of being overawed by the occasion of playing the Roosters early before clicking into gear in the second half to shock the defending premiers 20-14 on Saturday night.
The Panthers trailed 12-0 after 20 minutes with momentum in the visitors' favour until they began to claw their way back into the game.
The grind continued throughout the middle period of the contest before tries to Stephen Crichton and Viliame Kikau toppled the eastern suburbs outfit.
"At 12-0 down I thought it took us a while to get mentally in the game which is understandable when the Roosters are going so well and are clinical about what they do," Cleary said.
"It was definitely a slow start and the Roosters are a formidable machine when they get possession.
"To get back to 12-6 with Nado's try was a real boost for us and I thought that if we could start the second half well we'd give ourselves a chance and that's how it worked out."
Match Highlights: Panthers v Roosters
Cleary admitted his "heart was in his mouth" when Kikau left the field in the first half nursing what looked to be a serious shoulder issue.
But the Fijian international returned to the paddock shortly after the break to deliver the knockout blow in the 69th minute.
He opted to bench Kikau for the opening quarter of the game in a bid to manage his workload.
"I think he went numb down his arm and dare say it's never happened to him," Cleary said of Kikau's shoulder complaint.
"He didn't play any trials and has been a bit sore. We decided to take him out for the start of the game. It was always going to be intense at the start and we really wanted him on at the end.
"Playing the Roosters you need your strike guys so we wanted to have him on at the right time."
The Panthers are scheduled to play St George Illawarra next Friday night in round two at a crowdless Netsrata Jubilee Stadium in Kogarah but are waiting on the official go-ahead from NRL officials in regards to the ongoing coronavirus pandemic.
Cleary hoped a concrete decision would be made from the governing body on Sunday in the interests of everyone involved.
"It's just an hourly thing isn't it," Cleary said.
"My thoughts on the whole thing is to purely about people's health first.
"I just hope we don't keep doing band aid solutions. Maybe it's time to bite the bullet and get it sorted out.
"Other countries have gone into lock-down. I'm not a politician but it feels like we're changing our minds every hour. All it does is causes uncertainty."