Diego Costa's future will be under further scrutiny after Chelsea were drawn to face his former club Atletico Madrid in the Champions League. The Premier League champions, on their return to Europe's elite competition after a single-season absence, were pitted against Atletico, Roma and Qarabag, the first Azerbaijan side to reach the group stages, in Group C at Thursday's draw in Monaco.Costa has been an unauthorised absentee from London in recent weeks as he agitates for a move back to Atletico.The Brazil-born Spain striker and Chelsea are in dispute about the version of events.Chelsea say it was decided in January with Costa and his agent, Jorge Mendes, that the striker could leave in the summer. Costa, though, claims he is being forced out by head coach Antonio Conte.The situation is complicated by a transfer embargo which prevents Atletico from registering new players until January.Costa was playing for Atletico when Chelsea last faced them in the Champions League, in the semi-final in April 2014, and joined the Blues three months later. He scored a penalty as Atletico won 3-1 at Stamford Bridge, progressing by the same score on aggregate.Goalkeeper Thibaut Courtois was on loan at Atletico then and helped the Spanish side to the final at his parent club's expense.Former Blues Fernando Torres and Filipe Luis are now at Atletico.Antonio Rudiger will face his former club Roma, while the Blues must travel to Baku in November to face Qarabag.Tottenham have the unenviable task of facing holders Real Madrid and Borussia Dortmund in Group H, as well as Cyprus' APOEL."It's a great draw," Spurs striker Harry Kane said on tottenhamhotspur.com. "You want to test yourself against the best and Real Madrid are the best at the moment." Gareth Bale and Luka Modric will face their former club after in June helping Real become the first team in the Champions League era to win back-to-back titles.Scottish champions Celtic are also in a challenging pool, against Bayern Munich, Paris St Germain and Anderlecht in Group B.Manchester United, who qualified by winning the Europa League in May, received a favourable draw in Group A alongside Benfica, Basle and CSKA Moscow. United boss Jose Mourinho is seeking a win with a third club after victory in 2004 with Porto and 2010 with Inter Milan.Manchester City will face Shakhtar Donetsk, Napoli and Feyenoord in Group F, while f ive-time winners Liverpool will play Spartak Moscow, Sevilla and Maribor in Group E. It is only the second season that five clubs from one nation were in the group stage, after Spain had five representatives in 2015-16.Teams from the same association cannot face each other in the group stage and just two groups featured non-British teams.Juventus, runners-up in Cardiff in June, face Barcelona, Olympiacos and Sporting Lisbon in Group D.Group G features Monaco, Porto, Besiktas and Red Bull Leipzig.The final takes place at the NSK Olimpiyskyi in Kiev on May 26, 2018.The draw for the Europa League group stage, featuring Arsenal for the first time in 20 years, and Everton begins at 1100BST on Friday.