There is no substitute for quality substitutes.
Today in Perth Celtic proved their strength in depth off the bench.
There was a half time debut for Jeremy Toljan and the Dortmund fullback immediately gave the away side more energy down the right flank.
In the first half of the season, Celtic have lacked a physical presence up front since the departure of Moussa Dembélé.
The next few weeks will tell if Brendan Rodgers sees Oliver Burke as an orthodox winger or as powerfully imposing centre forward.
So far he has been deployed in the latter position.
The WBA player was brought off today as a tactical substitution and replaced with Odsonne Édouard.
It remains to be seen if the young Scot will fashion a permanent move to Celtic a la the player who came on for him.
When Édouard was replaced by Timo Weah only 12 minutes later it was out of dire necessity.
However, it proved decisive.
The young American contributed a brilliant assist and a finish from close range on the counter-attack to secure the points
Celtic went down to ten men when James Forrest hobbled off with all of the substitutes used.
A red card for Ajer saw the Hoops close out the match with nine on the field.
It was a bizarre turn of events when referee Willie Colum got it spot on.
The young Scandinavian was the author of his own dismissal.
Moreover, by that time Ryan Christie was reduced to a hobble in the middle of the park.
This was a victory through the pain barrier.
It is on days like this that teams without championship qualities can stumble.
Today Celtic did not.
The champions are unbeaten in 2019 and have yet to concede a goal this year.
It is title retaining form.
Rodgers knows that his guys just have to keep winning and that is 90% mental attitude.
Today was about attitude.
Next up Hibernian at Parkhead.