Celtic captain Jackie McNamara was livid after claiming he should have been given a penalty which might have kept Celtic's UEFA Cup dream alive.
Having gone behind 2-1 on aggregate to Sonny Anderson's early goal, the Scotland international fell in the area under a Rodolfo Arruabarrena challenge.
Italian referee Massimo de Santis waved away his appeals and Celtic tumbled out of Europe with a whimper. Villarreal eventually cemented their place in the last four with Roger Garcia's second-half goal but McNamara felt that first-half moment was decisive.
"I thought it was a definite penalty," he said. "I would have to see it on TV again but their left-back came across me and took me out as I went into the box.
"I looked around right away because I thought he was going to give it - mind you, we didn't get much out of him all night."
Martin O'Neill, who blamed Celtic's exit on mistakes and the absence of key players, said: "I haven't seen it back and the short answer is I don't know if it was a penalty, but you don't expect to get too many decisions away from home."
McNamara had harsh words for Villarreal, who he claims are nothing special despite a comfortable win last night.
Celtic knocked out Barcelona on route to the quarter-final and even though Villarreal had conquered Roma and Galatasaray, McNamara was adamant the Hoops missed a glorious chance of a semi-final spot.
"We are disappointed because everyone realised that this was a great chance for us," he added. "Villarreal are a good team but they're not great. I don't think they will win it and I have to say we have beaten better sides."
However, McNamara had to admit his team-mates never deserved a second straight semi-final appearance after some calamitous defending.
Bobo Balde was at fault for Anderson's opportunistic opener and Joos Valgaeren was punished by Garcia. "We lost bad goals for us," he said. "We have to deal with these situations a bit better.
"Their first goal came about when their guy ran between two of our own players to put the ball in, which is really disappointing."
Villarreal now face Spanish rivals Valencia for a place in a Gothenburg final.