Since beating Napoli 3-1 at the end of August to secure a 4-2 aggregate play-off win and subsequently book their place in Group H, Athletic have played six games in all competitions and managed to win only once.
After six league matches, Ernesto Valverde's men are languishing in La Liga with just four points, while the 0-0 home draw against Shakhtar Donetsk ensured an unspectacular return to Europe's elite club competition for the first time since 1998-99.
Athletic enjoyed a fine 2013-14 to finish fourth domestically, seven points ahead of nearest challengers Sevilla.
Despite their current poor form, Valverde is adamant that nothing has changed about his team since dispatching Napoli in the qualifiers.
"Honestly, we're the same, although we don't have the same breathing room," he told reporters after Saturday's 0-0 draw with La Liga newcomers Eibar.
"We're very thick in our interior movements. The team is working hard and fighting, but I think the things going against us are too much.
"We have a difficult time in our build-up play. We need to create danger. We need to find solutions and continue looking for them."
BATE do not go into Tuesday's game in particularly impressive form themselves following a four-game winless streak.
A 6-0 hammering at Porto – BATE's biggest away defeat in Europe – was sandwiched between a pair of home league draws against Torpedo Zhodino and Dinamo Minsk.
Despite three successive domestic draws, BATE remain three points clear of Dinamo at the top of the Belarusian Premier League, with the reigning champions enjoying a 23-match unbeaten league run.
BATE will not be able to draw much inspiration from their past meetings with Spanish opposition, though, having lost all eight previous matches.
The hosts will fancy their chances of ending that run as Athletic have only emerged victorious from one of their last six European games away from San Mames.
Ager Aketxe (broken toe) looks set to be the only player definitely missing for Athletic, while the out-of-favour Kike Sola is expected to sit out again and 18-year-old attacker Unai Lopez is pushing for a start.
Head coach Aleksandr Yermakovich, though, has no fresh injury concerns as BATE eye a first Champions League group stage win since beating Bayern Munich 3-1 in October 2012.
