Perhaps the South Africans saw his showcase in the Ashes as an outlier, rather than the new norm. Or perhaps they doubted the quality of the Englishmen facing up to him.
"We've played against Mitchell before. He's a fantastic bowler and he's playing really well, but it's not something that we haven't seen," Faf du Plessis remarked prior to the first Test, according to the ABC.
Trust us, Faf, this is something you haven't seen before.
It took Mitchell Johnson just four deliveries to shovel Du Plessis' pre-series words down his throat. More impressively, he disposed of Graeme Smith in only two.
Brutal, lethal, hostile, savage, rapid; regardless of your preference, those words all describe Johnson at Centurion on Thursday.
But is this speedster currently operating in rarefied air actually the world's No. 1 bowler as we speak?
The answer is an emphatic yes.
Those who watched Johnson's display on Day 2 against South Africa won't forget it any time soon. Smith was savagely bounced out, Alviro Petersen was softened up and nicked-off, Ryan McLaren saw his stumps shattered, while Du Plessis received the most brutal of throat-aimed deliveries; all 151 km/h of it ending up in Michael Clarke's hands at second slip.
With each wicket that fell, Johnson let go of a primal roar; his menacing demeanour indicative of a mindset that all fast bowlers aspire to, but few ever actually reach.
You could just picture Michael Holding, who was present at the ground, sitting smiling, rejoicing.
The figures, of course, don't tell half of the story. Whichever way you look at it, a return of 4/51 is impressive. Yet the hostility, the almost barbaric nature of Johnson's assault on the world's No. 1 side was something to behold.
Remember, this isn't a demoralised England outfit, trudging around Australia looking for the first plane home. No, this is the world's all-conquering side who have lost just once in their last 10 Tests on home soil. This is an opposition line-up containing three of the world's top 10 batsmen, including two of the top four.
AB De Villiers aside, they were blown away in a couple of hours.
Even when you take the emotion and the nature of Johnson's performances away, the raw numbers indicate he's currently the world's best. And it's not even close.
Test Bowling Records from July 1, 2013
*Minimum qualification: 5 matches, 15 wickets
Just let those numbers sink in. In just one more appearance over the latest seven-and-a-half month stretch, Johnson has 24 more wickets than Vernon Philander, the ICC's No. 1 ranked Test bowler. And that's before you even consider that both his average and strike rate are essentially twice as good.
Ditto for the comparison with Dale Steyn.
That's not to say that Johnson has been the standout bowler across the last two-to-three years. You could certainly make a case that he's not even close to being the best bowler since the start of 2013.
But right now, at this moment in time, no one is anywhere near his standard.
Yet perhaps the most overwhelming evidence of Johnson's ongoing effect on Test cricket is the perception the left-armer is forcing upon opposing nations.
Previously regarded as one of the country's greatest sides, England departed Australia in humiliation and despair; an entire era blown apart by just one man.
Despite their international influence, India are quickly being seen as a flawed outfit, with Johnson's feats highlighting the bowling deficiencies and lopsided strength of the Indian side. After all, that's the reason for the world No. 2 side's struggles in both South Africa and New Zealand, isn't it?
Even the South Africans suddenly find themselves scrambling for an answer. Prior to February, Graeme Smith's side were seen as next-to-untouchable for Test cricket's chasing pack.
One afternoon of brutality from Johnson has a whole bunch of English batsmen saying to their South African counterparts: "I told you so."
This is the effect Johnson is having.
On his own, the 32-year-old is reviving the rugged cricket played between the 1970s and 1990s, when names such as Marshall, Holding, Ambrose, Garner, Donald, Waqar, Akram and Akhtar sent shudders down the spine of batsmen worldwide.
On his own, he's hauling a recently broken Australian outfit back to the pinnacle of the sport.
Searing pace has—and always will—overwhelm opposing batsmen. Yet when that pace is mixed with unerring accuracy and a savagery in its delivery, it's truly spectacular.
So enjoy it while it lasts. Soak it up, revel in it, rejoice it. For this is fast bowling of the highest order, being delivered by the world's finest.