In the Champions League I don't think it makes any difference what their squad will be, nothing depends on it. The Champions League is about endurance and the ability to withstand any opponent, and for that you just need a balanced team with players who work well together and lastly to work on tactics. You can put together a star-studded squad, but it's unlikely to achieve greatness in the Champions League
Basically what you say is quite reasonable, considering the competition that occurs between clubs will be tighter, all teams try to prepare their best players for the upcoming match, therefore it is quite important for the Club to look for some players in the transfer market, maybe the Club can find at least one or two players who can strengthen the team, this can make a difference to the outcome of the match. But back again, no matter how good the player is, the main thing is teamwork, because a solid team can provide maximum results at the end of the match, one more thing that is no less important is the strategy applied by the coach.