By his own admission, time is running out for the 31-year-old Pollok ace to taste cup glory in the competition which back in 2011 delivered a career low point that he still frets over.
Diego was an influential midfielder in a Rob Roy side rated favourites to get the better of quarter-final opponents Dalry Thistle but the Rabs went crashing out in a 3-1 defeat.
And now he's vowing the pain has him determined to pull out all the stops and ensure Pollok do not come a cropper when hosting the visit of Beith in Saturday's fourth-round tie.
"Rob Roy had reached the semi- finals in the previous season only to lose 1-0 against Auchinleck over two legs but if anything the Dalry defeat hurt twice as much," McQuilken recalled.
"The difference was we were expected to beat them but on the day Dalry wanted it more and fully deserved their victory.
"Nevertheless it was a hard one to take and I'll never forget the awful feeling in our dressing-room afterwards."
The former Dumbarton signing could not have believed he would get another shot at the cup when the Willie Irvine-managed Pollok side he joined at the start of last season tumbled towards abject decline.
However, the appointment of his former Rob Roy gaffer John Richardson initially halted the Southside club's fall from grace before instigating a revival which sees Lok within touching distance of making it into the Emirates latter stages.
He said: "John has turned things around. It's a sign of the amount of changes that have taken place that only Brian McGinty, Darren Brady, Peter McMahon and myself remain from the squad he inherited .
"John has brought in tried-and-trusted players, many of whom are ex-Rob Roy who have done well for him in the past, and slowly but surely we are developing a great togetherness both on and off the pitch.
"I would call us a work-in-progress rather than the finished article but we have a fairly young squad so we might yet develop into a very good side."
Diego did not argue with the notion that the Newlandsfield clash might be decided by a flash of genius from either of the two top-notch strikers on show – Pollok's Carlo Monti or Beith's Kenny McLean.
He added: "Our fans call Kenny the scourge of Pollok because of his knack of scoring goals whenever he comes to Newlandsfield, so we'll need to keep tabs on him come Saturday afternoon.
"I'm sure Beith will look to do a similar job on Carlo, but if he is to shine then it is vital we do not leave him upfield on his own and isolated."
A seasoned campaigner lining up in the visiting ranks is full-back Gary Wilson.
Ironically, the nearest he came to achieving a cup winner's medal was as a Pollok player under Rab Sneddon in 2008 when the Glasgow cracks lost out to Bathgate Thistle in a semi-final clash.
The 33-year-old said: "That defeat still rankles as arguably the biggest disappointment of my career.
"Nobody knows better than me that I don't have many cracks left at the big one so I'll be giving everything on Saturday to try and clinch a last 16 place for Beith.
"Pollok are finding their feet again after an indifferent spell and I can see it being a very close game with little to choose between us."
Pollok's Willie Sawyers, who was left out as a precautionary measure from last week's line-up against Glenafton, has declared himself fit and ready which means only Brady is unavailable for selection.
Beith prepare to welcome back another Lok old boy in Calum Hardie who was attending a wedding when the Mighty went down 1-0 to Auchinleck last weekend.
Boss Frank Lynch said: "David McGowan picked up a groin strain which has him rated doubtful this week while Stuart Callaghan missed training earlier with a touch of flu and the hope is he will shake the bug off.
" Chris Mackie is also ruled out because of being cup-tied so we are thin on the ground for numbers."