Goals from debutant defender Dedryck Boyata and the Norwegian midfielder had the Scottish champions in a comfortable 2-0 lead in the first leg of the second qualification tie at Parkhead.
The opportunity to make the return leg in Iceland next week a whole lot easier came with around 12 minutes remaining after Johansen was felled in the box.
He wanted to take the resultant spot kick but second-half substitute Leigh Griffiths, nominated penalty taker, pulled rank before having his shot saved by former Motherwell keeper Gunnar Neilsen.
Manager Ronny Deila was infuriated by the incident, which evoked memories of Celtic's Scottish League Cup final win against Dundee United in March when James Forrest and John Guidetti argued over a penalty before the former's spot-kick was saved by Rado Cierzniak, and stressed he would sort it out.
McGarvey, who played for the Parkhead club between 1980 and 1985, told Press Association Sport: "Johansen was out of order
His behaviour reminded me of my school days where if you got brought down in the playground you took the penalty
"A penalty taker is under pressure, he needs concentration and Johansen affected that
He is completely to blame.
"All forwards need to score goals and Griffiths would have wanted a goal in the first game of the season.
"Griffiths is the best striker of a ball at Celtic but he hit at the right height for a keeper, although it was still a decent save.
"But it should be sorted out, everyone should know who takes the penalties and who should take them when that person goes off or is not playing.
"There should be no arguments."
McGarvey believes Griffiths, 24, has another reason to be irked.
The former Wolves and Hibernian player was top scorer at Parkhead with 20 goals last season, the bulk of which came after the turn of the year during a sparkling spell of form.
However, he was left on the bench against Stjarnan as Nadir Ciftci, recently signed from Dundee United for £1.5million, was given the main striker's role.
"Griffiths should have started because he was the top scorer last season," said former Scotland striker McGarvey.
"If I was him I would be annoyed
Ciftci has just come into the squad, he hasn't yet earned his place.
"I like Ronny Deila, I think he is a good manager, but if I was top scorer I would be looking to start the season and play until I stopped scoring.
"But I would have had both of them playing from the start anyway
I think you should have two strikers.
"One striker against an Icelandic team at Parkhead? - not for me."
Meanwhile striker Amido Balde has signed a two-year deal with French club Metz after being released by Celtic.
The 24-year-old former Portugal Under-20 international joined Celtic in 2013 for a reported £1.8million fee from Vitoria Guimaraes but failed to make an impact.
Source : PA
Source: PA