I don't think abramovich's departure is directly linked to the deterioration of the team, but yes, it was after that that some problems started. The squad costs a lot of money, large sums of money were spent on some players, and those players made absolutely no sense for the squad. For example - Mudrik, what has he done? One assist? Useless player for 100 million, yeah... I am not even talking about the choice of coaches, but you said about Zidane, will Chelsea have enough strength to negotiate with him? He's probably not interested.
Don't write off Mykhailo Mudryk, how many consecutive matches did he play that we're expecting him to kickoff his premier league career, he wasn't given enough time to adopt to the premier league and we have to blame the managers for this. Mudryk is a decent player and a fast one for that matter which means he'll be perfect on the left for Pochettino that loves counter attacking football. At Tottenham, we saw how Pochettino improved son and Harry Kane partnership and made them one of the strongest duo in premier league history. I believe Mudryk and the next striker that Chelsea will sign will have a lovely partnership.
Abramovich departure really affected Chelsea and it was untill the new owners took over the club that's when we realized it. Todd Boehly thought he could use money to solve the teams problem but he lacked the knowledge Abramovich had which was the staffs he surrounded himself with. Abramovich hired the best staff for each role in the club while Todd Boehly wanted to do everything himself and he messed it all up. Thankfully he has taken a step back and letting the club been handled correctly. Now he has given full control to the coaching staff, let see where the club heads from there.