The half-back came off with a knee injury towards the end of the first half of tonight’s 18-12 victory over Wakefield Trinity as Tigers ended a four-match losing run in Super League.
O’Brien left on crutches after landing awkwardly following a kick and looked upset on the sidelines.
Castleford - who defeated their derby rivals for the 15th successive time - face St Helens in the Challenge Cup final in a little over four weeks’ time.
Powell said: “Matty (Crowther), the physio, was checking his ACL just now and that looked alright.
“That’s always the worst-case scenario.
“Gaz felt a little better after the game and he was just so disappointed. “I’m praying he’s okay and he can get out there as soon as possible.”
Asked if the player - who has been so influential on their run to Wembley - felt like he was going to miss out, Powell said: “Potentially.
“He’s been in and out of the team consistently and picked up injuries q fair bit. He just can’t get a run going
“He said to me he can’t stay on the field. We’ll wait and see how the hyper-extension settles down.
“I would think he needs a scan on it.”
Castleford also saw second-row Jesse Sene-Lefao limp off with a hamstring strain.
On the game, which saw Castleford score tries through the excellent Pete Mata’utia, Jordan Turner, George Griffin and Michael Shenton, Powell said: “It was hard work.
“It was a tough game. You could see it was pretty attritional in terms of the heat.
“They are not used to it. We lost Gaz which forced a reshuffle and then Jesse came off.
“But the boys did well to win the game. We’re not all singing all dancing and we lacked fluency but we were better defensively and we needed a solid effort there.
“And we won. That’s the most important thing. We needed to get winning again.”
Wakefield boss Chris Chester lamented his side’s execution and said they “weren’t good enough.”
But they also saw centre Bill Tupou stretchered off with a worrying knee injury of his own
Chester said: “Bill’s in a lot of discomfort and I think it’s a patella tendon.,
“It didn’t look great. All the boys got around him in the dressing room and we have to make sure we give him all the support he needs.”