Voller well aware of Schurrle's threat
Leverkusen host Wolfsburg on Saturday knowing a win could propel them into a UEFA Champions League spot.
Voller, who has been at Leverkusen in his current role since 2005, finalised the paperwork to let Schurrle go to Chelsea in 2013.
However, after being deemed surplus to requirements by the London club, Schurrle is back in the Bundesliga with a vastly improved Wolfsburg side.
Schurrle has made an instant impact, but has rubbished reports that his partnerships with Bas Dost and Kevin De Bruyne - also an ex-Chelsea player - are similar to those of the 'magic triangle', formed by Stuttgart in their title winning season in 1996-97.
"We remain in contact, I'm delighted that he’s a World Champion," Voller told Wolfsburger Allgemeine Zeitung.
"London is a very difficult place to play with a lot of expectations.
"Nevertheless he is a strong player who brings Wolfsburg a lot of extra quality.
"At the time we had to sell Schurrle, we had other options but we also have a club to run, we had to sell to be economically on par.
"On Saturday it will be good for us because we are playing a team that plays - I expect a duel at eye-level and an open game."