By Karl Matchett.


German Bundesliga side Borussia Dortmund put themselves in a strong position in the UEFA Europa League last 32 with a 2-0 first leg win over FC Porto on Thursday, keeping Thomas Tuchel's side in with a shout of double cup success for the season.


While the latter stages of the Europa League are still some way off, BVB are already in the semi-finals of the DFB-Pokal and are just six points off top spot in the Bundesliga itself, with a home game still to play against leaders Bayern Munich. While the domestic league title might prove to be beyond them if Bayern don't slip up elsewhere, there is still plenty of reason to be optimistic about the team's chances of landing silverware in the two cup competitions, given how well key players are performing and the consistency being shown in results in general.


That overall consistency will be severely tested in the months ahead though, not just over the rest of the season with some tough fixtures, but also into summer and beyond with a number of important faces needing to renew their contracts-or be sold and replaced.


Presenting the defence


Since the days of Jurgen Klopp's successful team, the back four in place for Dortmund has been widely known and appreciated for the innate talents they bring: Marcel Schmelzer and Lukasz Piszczek at full-back, with Mats Hummels and Neven Subotic centrally. The latter has been ousted by Greek international Sokratis, but the remainder of the quartet is reassuringly familiar.


It's an important part of the side, and also a large part, but two see their contracts expire in little more than a year from now. Hummels and Schmelzer have both been linked with moves away for several transfer windows-often mere speculation but now surely with doses of realism-and, combined, make up the entirety of the left side of the defence. It's not simply individuals which matter in defence, but partnerships, and having to replace both components would be a real headache for the manager.


Add in Polish right-back Piszczek having only a further one year on his deal and it's clear that Dortmund need to decide promptly and act swiftly: keep faith with the defence which has served them well for a number of seasons and offer improved, long-term contracts, or else allow them to leave in the summer to avoid losing millions on Robert Lewandowski-esque situations of free transfers to rival clubs. 


If the latter is the chosen route, identifying not only talented individuals but players who can work together and flourish in Tuchel's system is of vital importance.


Maintaining the attack


The defence has remained largely intact over the years, but the same cannot be said of Dortmund's middle and final thirds.


Constantly circling sharks raid the Westfalenstadion for their top talents: Lewandowski, Shinji Kagawa, Mario Gotze and Nuri Sahin are the biggest-name departures over the past few seasons, with Marco Reus and Pierre-Emerick Aubameyang the players at the top of Europe's most wanted list this season.


Thankfully for BVB fans, those two are tied down to very long deals, into the next decade, but the other part of the attacking triumvirate is a different matter entirely. Henrikh Mkhitaryan is in the same boat as Hummels: just one year left on his contract and in the midst of his finest season at the club so far. The Armenian attacker started the campaign in blistering form and has barely relented, totalling 18 goals and 22 assists in 34 matches to date. He would, without doubt, fetch in the region of €20 million at a minimum this summer, even with a short time remaining on his contract, so it's imperative the Bundesliga club either cash in or renew. Clearly, he fits the framework of the attack, but if the funds he could raise would help renew not just his own position but also restructure the defence, Dortmund's decision might not be based on level of talent alone.


Dortmund face the same question in the middle of the park: how to maximise the value of, and adequately replace the skills of, Ilkay Gundogan and Sven Bender? Both have had severe injury problems, but both have also contributed an awful lot to the team.


Challenging for top honours, reaching the Champions League final and then battling relegation for a spell last season, before this year's surge back toward the summit of German football-this current Dortmund squad has been through an awful lot over the last few years.


This summer will pose the biggest and most important question at the club for a long time, though, as they look to renew one or the other: contracts of key players, or the overall face of the team.