Tottenham's 1-1 draw with Brighton on Wednesday wasn't especially significant in the wider scheme of things. But one notable aspect to the game was the return of Toby Alderweireld, starting a league game for the first time since October.

Last season it was almost unthinkable that Tottenham would be able to maintain their level of performance without Alderweireld, let alone without three of their first-choice defenders. But with Alderweireld absent for the meat of the campaign, Kyle Walker sold to Manchester City and Danny Rose out of favour, they have maintained a strong backline.

Only the two Manchester clubs have conceded fewer goals, and without Alderweireld they beat Manchester United twice, Arsenal, Chelsea, Borussia Dortmund and came very close to knocking Juventus out of the Champions League.

And for their chances beyond this season, that's just as well: a senior figure close to the Tottenham hierarchy told ESPN FC this week that it is "inevitable" that Alderweireld will leave the club this summer.

The suggestion that players like Alderweireld, in a similar manner to Walker last summer, will want to leave Tottenham is always an unpopular notion with the club's fans, but it's a logical inevitability that some of their top stars, for financial and/or football reasons, will look beyond the club.

That's the bad news. The good news is this season has displayed that, while losing these stars is hardly ideal, that they can not only cope but even thrive without them. The question then becomes: is that sustainable?

Last season, Alderweireld was arguably the best defender in the Premier League. Not only the quality of his defending, but also distribution from deep, such a crucial aspect of Mauricio Pochettino's play. He seemed intrinsic to their play.

But without him, Davison Sanchez has stepped up, fitting in with unexpected haste: the senior Tottenham figure described it as a "massive bonus" that, one or two errors you might expect and excuse aside, the young Colombian has been so impressive this season.

And he's been part of a defence that, in terms of personnel at least, bears little resemblance to last season's. Kieran Trippier has done well at right-back, while Ben Davies has kept Rose out of the team, that preseason interview in which he made clear he and others wouldn't hang around if they could increase their worth elsewhere, proving badly timed.

Toby Alderweireld's future at Tottenham is in considerable doubt. TF-Images/TF-Images via Getty Images

Part of the reason for this reasonably smooth transition might be that the replacements weren't purchased under any sort of significant pressure: they weren't necessarily signed as replacements. Davies was there anyway, Trippier too and he had almost replaced Walker at the end of last season anyway, and while Sanchez was their record signing, he wasn't purchased specifically to be a first-choice.

He had only just turned 21 when they finalised the signing, and the plan seemed to be for Sanchez to be eased in, possibly an option in a three-man defence, but not to be thrown in immediately. As it is he made his first start against Everton in September and only missed a handful of games since. Unburdened by the expectation of replacing Alderweireld, he has been able to seamlessly fill in for arguably the best defender in the Premier League last season.

"It's because he's so clever, and he's very humble, and he's very open to learn," said Pochettino of Sanchez in November. "He's a player when you tell him something his reaction is to be open, and be critical with himself, and that is a massive skill from a player, when he's so open to improve, and then the conditions he have are amazing to be one of the best centre-halves in the world."

But in any case, Spurs were confident of his quality before they spent such a significant sum on him. He was of course scouted extensively, and Pochettino sought advice from Juan Sebastien Veron, now chairman at Estudiantes and who is close to the the Spurs boss, who convinced him the young Colombian was the real thing.

Davinson Sanchez's form in his debut season has come as a real boost to Tottenham. Clive Brunskill/Getty Images

Another crucial factor has been the form of Jan Vertonghen, Those at the club, including Pochettino, have been impressed at how he has stepped up and taken responsibility this season, becoming the leader of the defence. That's been crucial with the changes all around him, and the slightly troubling increase in errors from Hugo Lloris.

The one thing knitting everything together is Pochettino's coaching. As you might expect, him being a former defender, he works extensively on defensive organisation, which is part of the reason why they're able to switch systems with ease. The players, Verthonghen in particular, put much of their success down to this coaching: indeed, if Pochettino coached Belgium, you would fancy their chances of World Cup success much more.

All of which is not to say that losing Alderweireld this summer won't be a blow: because he's still a terrific player, and it will strengthen the perception that Spurs are a stepping stone club, a place to hone your skills and then move to the absolutely elite level. If this keeps happening, they'll look like a souped-up version of Southampton.

Winning the Premier League, however much of a long-shot that is given the strength of their competitors, must be their aim: doing that will get tougher and tougher if they keep selling their best players.

But this season's defence has shown that this is a Tottenham team that, while Pochettino is there, they can evolve and cope with theoretically key players departing.