Billy Mehmet proved to be the hero for the home side when he grabbed the only goal of the game with just five minutes remaining of the replay to set up a quarter-final meeting with Celtic.

The fifth round replay was played in front of the television cameras and McGhee expected his side to put on a far better show.

He said: "I'm disappointed to lose the game but we probably deserved to lose.

"We were poor. We huffed and puffed and there was no lack of effort but there was no cohesion, fluency or quality.

"Our finishing was poor and we lost a shocking goal from a defensive point of view. We got what we deserved.

"I had high hopes given our recent form and the run we had been on. It was a TV game and I'm almost embarrassed, we let ourselves down badly.

"The supporters expected more, they expected us to get past St Mirren and play Celtic in the next round. I really thought we could get through tonight."

Buddies boss Gus MacPherson was thrilled to see St Mirren book their first win at their new home after snatching the last-gasp victory.

This was Saints' third clash at their new St Mirren Park stadium and MacPherson was relieved it proved to be third time lucky for his men.

"It might stop people from talking about not winning at home," he said.

"We have only had three games here and we had been playing well. We just had to remain confident.

"I think from the second half performance we just about deserved it. Motherwell had a few chances in the first half and looked dangerous but I think we deserved it in the end.

"We are looking forward to the quarter-final. It's a home game and teams will come here and enjoy playing here, none more so than Celtic.

"It will be a hard game but we'll deal with that when it comes."

St Mirren face a tough Clydesdale Bank Premier League game against Hearts on Saturday after the Edinburgh club refused to move the game to Sunday to allow extra recovery time for their rivals.

MacPherson said: "We were hoping it wasn't going to go to extra-time and we were trying everything possible to avoid it happening.

"We were getting men into the box and persevered and it was a fantastic ball from Craig Dargo and Mehmet did well to keep his composure and finish."

The Saints boss also played down a couple of incidents in the wake of the goal, when he appeared to be involved in an altercation with home fans behind the dug-out.

"That always seems to happen here and it was the same at Love Street," he said.

MacPherson was also involved in a spat with Motherwell assistant boss Scott Leitch and appeared to accuse him of unsporting behaviour after kicking the ball from his own number two, Andy Millen.

"I just told Scott to watch what he was saying," he said.

"You get caught up in these things but I was trying to be the peacekeeper for a change!"