The home side produced just two shots on target against relegation-threatened Heidenheim and created no big chances, while facing two themselves, and they only claimed all three points thanks to Emiliano Buendia's 91st-minute winner.

Alonso will be alarmed by the nature of his team's performance, especially as they had been eliminated from the semi-final of the DFB-Pokal four days earlier at the hands of third division side Arminia Bielefeld when they lost 2-1.

While the champions kept a clean sheet against Heidenheim, it was only the first time in eight games that they had prevented their opponents from scoring having conceded 14 goals in that time.

With just six matchweeks remaining in the top flight, Leverkusen must hope that Bayern drop points in at least three fixtures considering the Bavarians' goal difference of 54 is significantly greater than their figure of 29.

Die Schwarzroten have lost four of their past seven in all competitions – winning on three occasions – but they have triumphed in five of their last six in the league.

Though the hosts have suffered two defeats and a draw in three of their four most recent outings at BayArena, they achieved victory in 14 of their prior 16 at home, drawing their other two matches.

Union Berlin were largely excellent from a defensive perspective against Wolfsburg, but they were fortunate that Wolves striker Kevin Behrens somehow failed to equalise in the final minute of regulation time despite being less than a yard out from goal.

The visitors had not kept a clean sheet in their previous six games, but their form in the final third has improved considering they have scored six times in their last four, whereas they only managed 11 goals in their prior 17.

Die Eisernen will be looking to exact revenge for their 2-1 defeat at the hands of Leverkusen in the Bundesliga on November 30, but they are winless against their hosts in eight clashes, losing four and drawing four.

Boss Steffen Baumgart will come into the match confident given his team are undefeated in four matches, winning three times.

Union have also emerged as winners in their past two fixtures away from home – as well as three of their last four – but they lost all seven of the matches that they played on the road between October 30 and January 26.

Bayer Leverkusen possible starting lineup:

Hradecky; Mukiele, Tah, Hincapie; Frimpong, Xhaka, Andrich, Grimaldo; Adli, Garcia; Schick

Union Berlin possible starting lineup:

Ronnow; Doekhi, Querfeld, Leite; Trimmel, Haberer, Khedira, Juranovic; Schafer, Hollerbach; Ilic