The on-loan Albion midfielder has been added to Chelsea’s squad for the league stage.
He was initially left out after making his season-long loan move from Albion.
The 20-year-old is now listed by UEFA in the absence of Dario Essuga, who suffered a thigh injury while on international duty recently and has undergone surgery.
Injury-related replacements are permitted subject to certain conditions.
Buonanotte made his Premier League debut for Chelsea on Saturday in the first half of their 2-2 draw with Brentford and was quite lively.
It is also understood he has performed well in training.
Chelsea open their campaign with a game at Bayern Munich tomorrow.
They then face Manchester United at Old Trafford in the Premier League before Albion visit Stamford Bridge a week on Saturday.
Matt O’Riley could feature in the Champions League this evening as Marseille play Real Madrid at the Santiago Bernabeu.
The on-loan Albion midfielder went on as sub just before the break as OM beat Lorient 4-0 on Friday.
He has travelled with Roberto De Zerbi’s squad to the Spanish capital.
Attacking midfielder Hamed Junior Traore has remained in France due to injury but O’Riley is expected to start the game on the bench.