He recently passed 450 Blues appearances and started his 300th Premier League game in the draw with Brighton last week.
With more injuries and positive Covid tests in his squad, Tuchel had to make six changes to his team from the side which drew with Liverpool at the weekend.
Tuchel has stated that Lukaku was always the club's number one choice during their pursuit of a striker last summer.
The manager also confirmed that the Belgian striker is in contention to return for the Blues in Wednesday's Carabao Cup game.
Lukaku gave an interview to Sky in Italy in which the Belgian said he was "not very happy with the situation" at Stamford Bridge.
The hosts led from the 28th minute until the 90th, but an injury-time header from Danny Welbeck shocked the home crowd.
Danny Welbeck’s last-gasp header in front of the Brighton fans meant Chelsea were left ruing missed opportunities earlier on in the game.
Chelsea defeated Aston Villa 3-1 in the Premier League fixture on Sunday.
Chelsea conceded a 94th minute equalizer at Krestovsky stadium to settle for a 3-3 draw against Zenit and surrender top spot in Champions League Group H on Wednesday.
Christensen has gone from strength to strength under the German head coach, but doubts remain over the future of the defender.