WHERE would Brendan Rodgers and his strangely out-of-sorts Celtic side be right now without Leigh Griffiths and his goals?

There is no doubt that the Parkhead club merit their place in semi-final of the competition they have won in the last two seasons given how they dominated this last eight tie in Perth.

Yet, the defending champions still, due to a combination of the outstanding play of St Johnstone goalkeeper Zander Clark and their own poor finishing, were unable to edge in front.

As the game wore on, a team that has often disappointed in Europe and struggled domestically this term ran the very real risk of suffering a shock defeat and an exit from the competition.

When David Wotherspoon went close with a shot from the edge of the Celtic area in the 73rd minute there were a few anxious onlookers in the travelling support not to mention the away dugout.

Yet, then Griffiths, as he had done against Rosenborg in his side's opening Europa League group game seven days earlier, popped up with a vital late strike to secure the narrowest of victories.

With just seven minutes remaining, Kieran Tierney burst down the left flank and cut the ball back into the St Johnstone box where James Forrest met it with a downward header.

Scott Tanser blocked the Forrest effort - with what looked suspiciously like a hand ball - only for Griffiths to pounce on the rebound and drill a low shot into the net.

Dedryck Boyata, the Celtic centre half, received a straight red card from referee John Beaton in the closing stages for using foul and abusive language towards the match official.

But the defender's dismissal couldn't spoil the visitors' night. If they had failed to progress then their dip in form could quite easily have become a crisis. The relief at the end both in the stands and on the park was tangible.

Celtic, once again, had a lot to thank Griffiths, who has now netted three times in his last three games, for at the end of what was a nerve-wracking 90 minutes. His sudden return to form at a time when many of those around him are toiling has been timely.

Rodgers made a total of six changes to the Celtic team that had been beaten 2-1 by Kilmarnock in the Ladbrokes Premiership three days earlier. Boyata, Mikael Lustig, Kieran Tierney, Scott Brown and Griffiths were the only players to retain their places.

Scott Bain took over from Craig Gordon in goals – as he had done in the last round of this competition against Partick Thistle last month – and Kristoffer Ajer, Callum McGregor, Olivier Ntcham, Forrest and Edouard all came in.

Starting Edouard and Griffiths up front together for the first time was by far the most interesting development. Would making such a positive change work for Rodgers? It made little discernible difference to their performance level.

Edouard and Griffiths did link well on the half hour mark, the Frenchman supplying the Scot a few yards out with a diagonal delivery, but Clark denied the header with a decent reaction save.

It is early days in the partnership. But there appeared little chemistry or great understanding between the pair. It remains to be seen if the experiment will be repeated in future.

Tommy Wright kept faith with all but one of the member of the St Johnstone side that slumped to a 5-1 defeat to Rangers at Ibrox at the weekend. Ross Callachan came in for Matty Kennedy in midfield.

But his charges carried out his game plan to perfection without ever really troubling their opponents.

Bain, who was making his first appearance in five weeks, dealt with an ambitious Liam Craig effort after a Wotherspoon free-kick in the 18th minute. That aside, though, he had little else to do.

Ajer, who had missed both the Rosenborg and Kilmarnock matches last week with a virus, lasted little more than half an hour. He pulled up injured in the 33rd minute. Jack Hendry took his place in the centre of defence alongside Boyata.

Celtic should have taken the lead in added-on time at the end of the first-half. Ntcham sent McGregor clean through on goal with a perfectly-weighted through ball. But his team mate shot wide of the right post. He really just needed to get his effort on target to put Celtic ahead.

The Scotland midfielder knew he should have done far better and had passed up the best opportunity of the game. He looked to the sky in disbelief as Beaton blew the whistle for half-time.

McGregor, though, set about making amends for his miss at the start of the second-half. Clark did well to tip a deflected shot by him out for a corner just three minutes after the game restarted. Celtic then proceeded to lay siege to the St Johnstone goal.

Clark denied Ntcham and McGregor with fine one-handed saves while Boyata fired over and Griffiths hooked wide.

Rodgers had soon seen enough. He replaced Edouard with Lewis Morgan, who had made his return from a two-and-a-half month injury lay-off when he had come off the bench against Kilmarnock, in the 66th minute. The former St Mirren winger went close just a minute after taking to the field.

Griffiths did superbly to chest a high ball from McGregor between Joe Shaughnessy and Jason Kerr in the 73rd minute. His deft touch put him one-on-one with Clark inside the St. Johnstone box. Few in the 5,635-strong crowd would have backed the keeper. But the Celtic forward’s finish was poor and struck an outstretched leg.

Fortunately for the Scottish champions, he atoned with seven minutes remaining.