If Munster beat Edinburgh with a bonus point at Thomond Park in Round 5, Gloucester will have to match that feat at Perpignan and then see off the Irish giants in the West Country, bagging maximum points in the process and preventing Munster from taking anything back to Limerick, if they are to finish top of Pool 6.
Director of rugby Nigel Davies admitted that the Round 4 reverse was a missed opportunity for his men but Twelvetress went even further when refusing to hide his disappointment at a result that could yet cost Gloucester a Heineken Cup knockout spot for the first time in six seasons.
"It's embarrassing to be honest. It hurts and the guys were gutted in the changing rooms," England centre Twelvetrees told The Citizen.
"The boys want to work hard for each other and do the shirt proud but it comes down to a bit of mentality, decision making and calmness that is letting us down and you can't get away with that in Europe.
"We weren't smart enough when we needed to be. We left some chances out there and our discipline let us down. It's about executing, when to pass and when not to pass, discipline in the right areas and being patient in the twenty-two.
"Edinburgh came down and made a nuisance of themselves in tough conditions. But we had opportunities to score and take points but we didn't do that. Good teams put chances like that to bed."
Gloucester are now five points adrift of Munster with two game left to play, with the 2006 and 2008 Heineken Cup winners topping the table with 14 points and Gloucester sitting second on nine, a point clear of Edinburgh and two ahead of Perpignan.