United's attackers were guilty of missing a host of chances in the quarter-final first leg in Belgium and will now go back to Old Trafford only marginally ahead on away goals when it could have been so much more.
"It's frustrating," Carrick told BT Sport.
"We were so in control but got sloppy, we have to kill teams off. We should have won that game. They had one chance and scored a header.
"We need to be more ruthless, it's as simple as that. We were too comfortable. We got sloppy.
"It's not the end of the world but we are better than that. It's that little edge that makes the difference in the end, but we'll look forward to the next game."