Castleford knew they would secure a top-six spot with victory at Hull College Craven Park and they did lead 19-12 approaching the hour mark on Saturday.
However, KR struck with tries from Matt Parcell and Kane Linnett in quick succession to take control.
Tigers, who drop to seventh, must now beat Warrington Wolves at Wheldon Road in their final regular round game on Thursday to qualify.
Powell said: “Obviously we’re disappointed we lost the game.
“It was a game we were in the battle for but I don’t think we ever played that well really.
“It’s tough to take. It was a big game for us and we patched the team up a little bit with obviously Pete (Mata’utia) playing at half again.
“There were a couple of areas where we didn’t quite get our game right but we live to fight another day.
“It will be a tough day but we just have to find a way on Thursday.”
Rovers move up to fifth and they will make the play-offs if Cas lose against Warrington, the club Powell joins at the end of the season.
However, if Tigers win, Rovers must win at Leeds Rhinos - the other side in a three-way battle for the last two spots - at Headingley on Friday.
Cas led 13-12 at the break after Niall Evalds scored the first of his brace and Derrell Olpherts also crossed with Danny Richardson converting twice and slotting a drop goal.
However, Powell’s side had done so against the odds.
“We just didn’t have any territory,” he conceded.
“We had one good ball set.
“We just didn’t have any possession in good field position.
“We dropped it on the fourth tackle, a drop-out bounced straight out...
“We were in front at half-time which I was pleased about but I don’t know how it happened given territory -wise the swing was so big against us.
“And then obviously we had a seven point lead in the second half but I just thought our game management was poor.”
Shaun Kenny-Dowall appeared to knock Jordan Abdull’s high kick forward in the build-up to Parcell scoring on the ex-Leeds hooker’s 100th Super League appearance.
Powell said: “That try is not a try. That swung the game a little bit. Referees have just got to do their jobs a little bit better.
But so do players and coaches.
“It was that period that turned the game against us and we just didn’t have the strike or energy to find a way back into the game.
“Hull KR just about deserved to win the game and credit to them. They’ll roll on to Leeds.”
Referee James Child did not consult the video referee about the Parcell try which did confuse Powell and Abdull’s 40/20 in the next set led to Linnett putting Rovers in front.
Asked if he thought the video referee should have been used, Powell said: “It would make sense; that’s what it’s there for.
“I think it’s a poor decision. It was difficult for the naked eye to see so in that instance I think it has to be used but I’m not going to blame that for the result.
“We just have to look at ourselves for the reasons why we didn’t win that game.”
Adam Milner was excellent at filling in at hooker once more for the absent Man of Steel Paul McShane.
However, Cas hit problems when replacement hooker Jacques O’Neill also had to depart himself.
“I thought Adz was great in that first stint and then Jacques O’Neill came off with a hamstring strain,” explained Powell.
“That put Adz back on which stopped me doing what I wanted to do.
“Adz is battling away there at nine. Paul McShane is our regular hooker and he’s missing at the moment.
“That has a decent impact on the way that we play but again every team has injuries and they are managing their way through it at the moment.”
He said McShane might be available to return against Warrington and veteran prop Grant Millington could also return while fellow front-row Liam Watts will be back from a two-game ban.