After the 0-0 pre-season draw with Porto, Magpies boss Rafa Benitez explained that it's a cruciate ligament injury that has kept Frenchman Lejeune out of the side, and with the player needing surgery he looks set for a lengthy spell on the sidelines and will possibly miss the entirety of the 2018/19 campaign.
That leaves Newcastle a little bare in the heart of the defence, and recruiting an additional centre back must now become a priority before deadline day passes on August 9.
Newcastle have already signed Swiss international Fabian Schar this summer, who will likely start the season partnering Jamaal Lascelles. However, beyond those two, the club are left with just Ciaran Clark as experienced cover. Young defender Josef Yarney has had run-outs in pre-season and he is currently the fourth choice, but it remains to be seen whether the player would be ready for Premier League football.
For safety's sake, it would be wiser not to risk needing him. Introduce him at the club's own pace, rather than risk him being thrown into the fire after just a couple of injuries.
Added cover with top flight experience and quality, therefore, has become somewhat of a priority.
With Mike Ashley notoriously tight-fisted in the market, it almost goes without saying that any addition need not be particularly outstanding, either in terms of quality or fee - which should make recruitment a lot easier. While that may seem a little depressing, Newcastle aren't in need of a starting central defender, just some cover should injuries start to mount up.
As such, the Tyneside club just needs someone on reasonable wages who is happy to take their place on the bench and wait for an opportunity. It's purely a case of not wanting to be caught out cold, as there has been enough warning with this Lejeune injury that it would be criminal to fall because the club is unprepared.
Newcastle should have the money to make a cheap signing too, without it affecting the existing transfer plan this summer, now after Aleksandar Mitrovic has completed his move to Fulham for at least £20m.
But if money is tight, even a free transfer could be a good option.
The Premier League season, especially during the Christmas period, is extremely busy and intensive and with a cruciate ligament problem Lejeune is looking at months out of action, so Newcastle are going to need the backups and safety net of a spare defender.
Newcastle do have one other option, which would be to move Paul Dummett more centrally - where he has played in the past - and utilise the out-of-favour Achraf Lazaar as full back cover in place of the Welshman.
Ultimately it comes down to Benitez and what he feels is necessary, and he can fit into the tight budget, but he too seemed to understand the foolishness of risking the upcoming Premier League season without enough defensive cover.