The Swiss hosts have work to do if they are to progress, with their opponents arriving with a 1-0 aggregate lead from last week's first leg.
Match preview
Having missed out on the Swiss Super League title by 14 points last season, Basel had to settle for a second-placed finish and a second-round entry into Europa Conference League qualifying, and they began with a tie against Crusaders.
A 2-0 home win thanks to goals from Dan Ndoye and Adam Szalai was enough to see the RotBlau progress, as the second leg was drawn 1-1, before they squared off against Brondby in the third round.
That contest went down to the wire, as Basel cancelled out a 1-0 deficit from the first leg to win 2-1 at home and take the game to extra time and eventually penalties, which they prevailed in as Andy Diouf, Ndoye and Darian Males all converted their spot kicks.
Such a triumph saw them book a playoff tie with CSKA Sofia, and they headed into the first leg last Thursday on the back of a disappointing league defeat to Lugano, leaving them eighth in the Super League and winless from their opening four outings.
Following the narrow defeat in that first, Basel were able to put a confidence-boosting cup win on the board last time out, thrashing Allschwil 5-0 away from home as Males hit a hat-trick alongside goals from Bradley Fink and Szalai, and they will now hope to bring the momentum from that result into Thursday's contest and overturn the deficit on home turf.
Sasa Ilic's side, meanwhile, will make the trip hoping to book a place of their own in Europe's tertiary competition after picking up a hard-fought home win last week.
Last season's second-placed side in the Bulgarian top flight entered qualifying in the second round and began with a 4-0 victory over Makedonija, with all four goals coming in the second leg courtesy of Jurgen Mattheij, Georgi Yomov, Lazar Tufegdzic and Duckens Nazon.
They would then meet St Patrick's Athletic in the third round, and, after losing 1-0 on home turf, they were eventually able to turn the game on its head on their travels and progress with a 2-1 aggregate win thanks to goals from Mauricio Garcez and Ivan Turitsov.
On the back of a league triumph over Septemvri, the Reds hosted Basel in the first leg of their playoff tie last week, and they took a step closer to the group stage with a 1-0 victory, as a Sergio Lopez own goal made the only difference at the Vasil Levski National Stadium.
While they face a particularly tough test away at the Swiss giants, Ilic's side will hope to take advantage of Basel's relative struggles so far this season and hold their lead to progress.
Basel Europa Conference League form:
Basel form (all competitions):
CSKA Sofia Europa Conference League form:
CSKA Sofia form (all competitions):
Team News
Veteran striker Adam Szalai was reduced to a substitute appearance for Basel in the first leg last week, and he should come in from the outset on Thursday, with new signing Andi Zeqiri also vying to feature.
There will be support for the front line from midfield, too, with Liam Millar and Dan Ndoye set to operate on the wings while Darian Males has enjoyed a positive start to life with the Swiss outfit after signing on loan from Inter Milan and could also come in from the outset.
Behind him, Andy Diouf, Wouter Burger and Fabian Frei will likely find themselves fighting for two spots at the base of midfield with Kasim Adams and Arnau Comas set to partner up at the heart of the defence, in front of former Borussia Dortmund goalkeeper Marwin Hitz.
CSKA Sofia will likely come into Thursday's game with an unchanged starting XI from last week, although the likes of Lazar Tufegdzic will be vying to come in from the outset after featuring off the bench in that game.
Mauricio Garcez remains a key man going forward, with the winger boasting six goals and two assists in nine appearances across Conference League and domestic action since his loan arrival from Brusque.
Brayan Moreno and Duckens Nazon will battle to operate as the lone striker, while Amos Youga is a relative mainstay in a midfield anchor role.
Basel possible starting lineup:
Hitz; Lopez, Adams, Comas, Pelmard; Burger, Frei; Millar, Ndoye; Zeqiri, Szalai
CSKA Sofia possible starting lineup:
Busatto; Turitsov, Koch, Mattheij, Petrov; Lindseth, Youga, Vion; Yomov, Moreno, Garcez
We say: Basel 2-0 CSKA Sofia (Basel win 2-1 on aggregate)
CSKA Sofia performed admirably to take a lead in their home leg and will be desperate to stay ahead, but we see the hosts having slightly too much and overturning the deficit in Switzerland.