As has been the case in many of their recent matches, Rangers again dominated for large chunks of the game without offering any real cutting edge in attack and were undone by two second-half goals from Steven Milne.
Le Guen, certainly, could not be accused of a lack of attacking intention. He had set his team out in the 4-4-2 formation many people had been hoping to see and paired Kris Boyd and Filip Sebo in attack for the first time this season.
With Nacho Novo and Thomas Buffel operating on either flank it meant Rangers effectively had four strikers with which to break down the St Johnstone defence although real goalscoring chances would be few and far between.
The Perth side, currently topping Division One, also began with two up front, perhaps indicative of their impressive start to the campaign and a porous Rangers defence seemingly allergic to keeping clean sheets.
The latter fact, along with the wintry conditions, perhaps contributed to a crowd of only 31,074 who stood almost to a man to applaud the life of Rangers legend, Bobby Shearer, who died earlier this week.
Celtic’s ejection from this competition at the hands of Falkirk the previous night should have provided extra motivation for Le Guen’s men to brush aside St Johnstone and progress to the semi-finals but there was little to differentiate the teams throughout the 90 minutes.
As has been the case in many matches this season, Rangers had the bulk of possession without causing the opposition keeper much in the way of palpitations.
Sebo muscled his way to the periphery of the box before thundering an effort over before Buffel found the side netting from a near impossible angle as St Johnstone comfortable contained a toothless attack.
At times Rangers’ play was so laborious and unseemly that attacking players literally fell over each other as they tried to find space in the visitors’ box.
From one such unsightly scene St Johnstone rushed forward and only a brave sliding tackle from Steven Smith prevented Steven Milne from launching an effort towards Allan McGregor.
As half-time approached the only noise evident from the half-empty stands were murmurs of discontent as Rangers again made heavy weather of beating seemingly inferior opponents.
It would get worse for the home support just six minutes after the re-start when St Johnstone went in front.
The warning signs had arrived just minutes earlier when Jeremy Clement’s misplaced pass fell straight to Simon Mensing who found Jason Scotland with a searching ball.
The former Dundee United man strode in on goal but was unable to find a way past McGregor who did well to block the shot.
Respite though for Rangers would be brief. From the following corner, Milne executed the perfect overhead kick that flew past McGregor before he could even move.
Le Guen’s response was to haul off the ineffective Sebo and Buffel and replace them with Dado Prso and Lee Martin, the latter soon causing more problems with precision set-piece deliveries than his team-mates had managed in the previous hour.
Losing the goal at least seemed to wake Rangers from their slumber although neither Hutton nor Boyd could direct headers on target from Martin crosses.
When Prso then blasted over from 12 yards after being set up neatly by Boyd and then Novo the Ibrox denizens wasted no time in roaring their frustration. It was turning into one of those days. Again.
Rangers were virtually camped in their opponents’ half but would again be undone by their own fallibility.
Clement, rushing into the box to support his team-mates, lost possession to Steve Anderson whose long ball set up a sprint between Milne and Brahim Hemdani.
It was a race the St Johnstone man won easily and he made full use of possession to rifle a low shot past McGregor.
Rangers’ night was summed up four minutes from time. Prso, in acres of space after good play by Martin, elected to find Boyd rather than shoot.
The former Kilmarnock man’s effort seemed to dribble over the goal line before it was hacked away by Andy Lawrie only for the referee’s assistant to shake his head and wave play on.
RANGERS (4-4-2): 22 McGregor; 20 Hutton, 5 Papac, 7 Hemdani, 26 Smith; 10 Novo, 8 Clement, 11 Rae (Adam 69), 4 Buffel (Martin 54); 15 Boyd, 23 Sebo (Prso 54)
Subs not used: Klos, Svensson
Booked: Smith (71), Prso (86)
ST JOHNSTONE (4-4-2): 1 Halliwell; 2 Lawrie, 5 McManus, 6 Anderson, 3 Stanic; 4 Mensing, 7 Lawson (Sheridan 89), 8 Hardie, 11 Sheerin; 9 Milne (Jackson 90), 10 Scotland (MacDonald 83)
Subs not used: Cuthbert, Dyer
Booked: Stanic (57)