Celtic's domestic struggles continue. In any other season, the claiming of only eight points from their opening five Scottish Premier League matches would trigger panic and inquest around Celtic Park.

These, however, are curious times with no Rangers around to capitalise on their city rivals' shortcomings. No other SPL team have offered a strong challenge to the defending champions. It seems legitimate to link such matters to Celtic's sluggishness, which was evident for the latest occasion in Perth. The challenge for Neil Lennon is to lift his players suitably for Benfica's visit to Glasgow on Champions League business on Wednesday night.

The Portuguese scout who attended this fixture would not have departed with much concern but Lennon wasted little time in voicing his.

"It was a needless performance, unacceptable," Celtic's manager said. "We could have played anybody today and we would have got beaten. There were too many stray passes, we played like strangers at times.

"There is definitely a mindset there that we need to get to the bottom of and change quickly. If they are cruising through games then they won't play. That is not what they are paid for.

"Their goals were awful. We never laid a glove on St Johnstone for either of them. There was a softness about us."

Few could argue St Johnstone deserved a first win in 15 outings. Celtic's key players, including James Forrest, failed to perform with the hosts' showing all the more admirable given the flu outbreak that swept through McDiarmid Park and led to the cancellation of training earlier this week. Lennon handed a first start to Miku, Celtic's on-loan Getafe forward, but the Venezuela international had no impact on proceedings whatsoever.

And yet Celtic had started as the brighter side. Kris Commons sent them in front with a low finish after a brisk move involving Scott Brown and Gary Hooper.

Hints of a St Johnstone reply arrived from a Gary Miller free-kick, smartly saved by Fraser Forster, and an effort from the excellent Peter Pawlett that Kelvin Wilson blocked at close-range.

An equaliser duly arrived, with Celtic's lax defending key to the goal. Rowan Vine played a pass infield to Grégory Tadé; the Frenchman had been afforded more space than he could probably believe but lashed a first-time shot beyond Forster rather than worry about that.

Controversy followed. Celtic were denied what looked like a clear penalty as Steven Anderson bundled over Commons. If the award had been made, the St Johnstone defender would also surely have been sent off.

"Why would he [Commons] go down? He was clean through on goal," said Lennon, not unreasonably. "It was a penalty and a red card."

Pawlett scooped over the crossbar on the stroke of half-time, amid signs of a Celtic wobble.

St Johnstone were themselves unfortunate not to have been awarded a 62nd-minute penalty when the Celtic defender Charlie Mulgrew barged Pawlett off the ball. The referee, Euan Norris, will point towards a shoulder charge but it seems highly likely he would have awarded a foul if the incident had occurred elsewhere on the field. Steve Lomas, the St Johnstone manager, described Mulgrew's challenge as a "WWF bodycheck".

No penalty was required for St Johnstone to claim three points. With 10 minutes to play, Vine curled home a sublime shot after collecting Nigel Hasselbaink's pass. The substitute Tony Watt could have salvaged a point Celtic would have hardly merited but had his effort in the dying seconds was saved by the feet of Alan Mannus.

"When you are playing well and not getting results, you could get despondent but the boys never became like that," said Lomas after St Johnstone's first league victory of the season.

"It was reward for a lot of good performances and not a lot of results in the first five games. I believe Celtic will go and beat Benfica but I don't want to take any credit away from our boys – they were superb."