Barcelona will find out its opponent as coach Tito Vilanova, who has led his team to a record start of 15 wins in 16 Spanish league matches, undergoes surgery on this throat before beginning a course of chemotherapy and radiation therapy.
The club said Wednesday that 44-year-old Vilanova's treatment is expected to last six weeks until just ahead of the next Champions League fixtures in mid-February.
Barcelona, Madrid — and their star forwards, Lionel Messi and Cristiano Ronaldo — are joined in the draw by Champions League debutant Malaga and Valencia in a show of Spanish supremacy. Spain is the best represented of nine countries with teams in the knockout stage.
Germany has last season's beaten finalist Bayern Munich, plus widely admired Borussia Dortmund and Schalke. All won their group and will be seeded in the draw, assured of playing a runner-up at home in the second leg.
England's recent dominance of world football's most prestigious club competition has faded, and only Manchester United and Arsenal will represent the Premier League.
Chelsea's failure — in a group that saw Juventus and Shakhtar Donetsk advance — means the Champions League has never been successfully defended since the European Cup's pure knockout format was abandoned in 1992.
AC Milan, in 1989 and '90, was the last back-to-back winner, and seeks an eighth European title despite off-season cost-cutting sales including forward Zlatan Ibrahimovic and defender Thiago Silva to Paris Saint-Germain.
An Ibrahimovic reunion is perhaps the most intriguing potential pairing.
Fuelled by its Qatari owners' lavish transfer spending, PSG advancing from the Champions League groups for the first time since 2001 has given the competition a different look this season.
Just five of the 16 teams in Thursday's draw reached this stage one year ago: AC Milan, Arsenal, Barcelona, Bayern and Madrid.
United returns to the last 16 after its group-stage elimination by Basel last season, and Juventus last reached the knockout stage four years ago.
Celtic and Galatasaray will give Scotland and Turkey their first representation at this stage since 2008.
Despite beating Barcelona at home last month, Celtic shapes as arguably the weakest team of the 16.
Still, it could be drawn against Malaga, which UEFA ranks No. 62 in Europe, the lowest of the remaining teams despite being unbeaten in eight matches in this season's competition.
UEFA rules mean clubs cannot be paired on Thursday with a previous group-stage opponent, or another from its home country.
Group winners are seeded and will play the first-leg match away on Feb. 12, 13, 19 or 20. Return matches are played on March 5, 6, 12 and 13.
The final is on May 25 at Wembley Stadium — a possible omen for Barcelona whose four European titles were won either at the London venue (1992, 2011) or against English teams (Arsenal in 2006, Man United 2009).
Seeded clubs: Barcelona, Bayern Munich, Borussia Dortmund, Juventus, Malaga, Manchester United, Paris Saint-Germain, Schalke.
Unseeded clubs: AC Milan, Arsenal, Celtic, Galatasaray, Porto, Real Madrid, Shakhtar Donetsk, Valencia.