Bruised and battered opponents will confirm this as they wince at the memory of the blond bludgeoner.
By contrast, Kelvin Wilson is a footballer first and foremost, relying on his skill and pace to intercept and create with the minimum of contact.
Nevertheless, the Swede and the Englishman are united in a common cause – to make Celtic's recent defensive frailties a blip and not a long-term problem.
Assistant manager Mjallby takes special responsibility for coaching the defence.
So watching them concede eight goals in their last three games against Juventus, Ross County and Aberdeen has cut him to the core.
The Italian champions were always going to test the Celtic backline, especially as the Hoops were 3-0 down from the first leg.
But, for Mjallby's liking, the teams from the north of Scotland found it far too easy to breach Celtic's defences, especially when the men who have looked so solid this season – Wilson and Efe Ambrose – were in their posts.
"The good thing is the team has defended really well in Europe," said Mjallby, before adding, "Maybe that's why we have been a bit disappointed lately that, domestically, we have not defended as a team that well.
"That's something we need to get back to."
Wilson – who was badly exposed in Dingwall and again by Aberdeen's battering-ram striker Josh Magennis last weekend – concurs.
Fortunately for him, he has on hand an experienced figure who knows all about the pressure involved in being a cornerstone of a successful Celtic side, even if their methods could not have been more contrasting.
Wilson said: "When you have coaches who have played in your position, like Chris Fairclough at Nottingham Forest, you can go to them and ask them advice.
"Or, when they give you advice, I think you listen.
"But I think me and Johan were different types of centre-backs. He was a no-nonsense centre-half. I try to be more of a footballing centre-half."
That's a style which has won Wilson plenty of plaudits this season, having taken his first year at Celtic to find his feet and his form.
Mjallby must be tempted to tell Wilson to find row Z when the need arises, rather than try to play his way out of trouble.
But he appreciates the 27-year-old's qualities and footballing philosophy, and recalled how he was instantly struck by his ability.
Mjallby told Celtic TV: "Me and the manager went down to watch him more than three years ago when he played against Scunthorpe.
"Gary Hooper was playing, so it was a good game in which to watch both of them.
"The manager knew Kelvin already because he played with him at Forest.
"I had my first look at him, and I thought straight away he would be a success for us.
"He struggled a little bit to settle in. It's a bit different, the game up here, where you have to compete a little bit more in the air and you can't always depend on your pace.
"Obviously, Kelvin is perfectly set up for the European game.
"But he has improved a lot in the air. He's much stronger now and more of a leader.
"He can improve even more, but he is so quick and his recovery pace is brilliant."
That has been shown to best effect in the Champions League run, and Mjallby reckons that has put Wilson into contention for the shortlist for Player of the Year, along with Fraser Forster, Victor Wanyama and Gary Hooper.
He said: "Especially when you look back to the European campaign, I have to say Kelvin has been a standout.
"He has been unbelievably good in Europe."
Which makes the current dip in form all the harder to fathom or, if you are Mjallby and Neil Lennon, to accept.
Wilson has his own theory why performance levels have dropped.
"We are human beings, so we're obviously going to get tired," said the man who has played in 42 games this season, twice the number he managed last time around.
"Some people say it is a mental thing. But it's not, it's your body."
He went on: "It's been a tough campaign with plenty of games with the European run and the cup runs.
"But it's nothing we are going to moan about."
After his stop-start, injury- affected debut season at the club – which followed five months frozen out at Nottingham Forest after signing a pre-contract agreement with Celtic – it has certainly been a test of Wilson's strength and endurance.
But despite his recent lapses, overall, he is considered one of the most improved players in Lennon's squad.
He believes he knows why that should be. "I've probably played a bit more and stayed injury-free," Wilson told Celtic TV.
"Things off the pitch are good as well, so that's the main thing."
He added: "You improve with age. You improve yourself by games.
"And I think the European campaign has probably improved me, playing against world-class players."
Wilson hopes he will be rewarded with a league and cup double to add to the championship medal he collected last season.
There could also be some personal awards in the form of the club Player of the Year prizes, while Charlie Mulgrew has tipped him to be a contender to succeed him as the SPFA's top performer.
However, Wilson believes there is serious competition from a clutch of his team-mates and said: "There are quite a few in our changing room who are up for that.
"Fraser Forster is one. If it was not for him, I don't think we would have had our run in the Champions League.
"He's been brilliant since I came here, and him being involved with England does not surprise me.
"Take nothing away from Joe Hart, he is world class.
"But if Joe was not there, I think Fraser would be pushing to play."