"We feel great relief in all honesty. It could have gone either way, it was so close and we're very pleased to come away with the points."
Coppell hailed his matchwinner and also had words of praise for Wednesday, whose skipper Richard Wood had a last-gasp header cleared off the line before Owls goalkeeper Lee Grant saw his header brilliantly saved by Reading counterpart Adam Federici moments later.
Coppell added: "Longy is my best impact sub. He showed a lot of composure to score from that angle when he had a lot of time to think about it.
"Credit to Wednesday, they could easily have got something there. It was real desire on their part and commitment on my players' part to keep it out.
"It was like two heavyweights slugging it out in the 15th round. It was a great save on the line and then a great save from the goalkeeper. I think it could easily have been a draw."
Reading moved second as a result of only their third win this year and Coppell feels the race for a top two spot is wide open.
He said: "It's now down to somebody to earn the right to play in the Premiership next year.
"If you put together a run of six, seven, eight victories, or eight positive results then you could be playing your football at a higher level next year.
"Nobody's going to trip over the finish line. You've got to be going at full pelt to be playing in the top division."
Brian Laws was not happy with the referee.
"I don't want to give too much coverage to an official because I don't think he deserves any coverage today.
"The least we say about him the better. I'm more concerned with the effort of the players, who've put in a really hard shift against a really good side.
"Reading are my prediction for promotion automatically. I've said that for months and I won't change my mind on that one."