After an action-packed schedule over the past 18 months due to the coronavirus pandemic, teams will be hoping not to overload their players as they head into another gruelling winter period.
Like last season, substitutions could be crucial for each side in the Champions League as they look to avoid any serious injuries and fatigue.
How many substitutions can teams make in the Champions League this season?
As a result of the pandemic last season, teams were allowed to make six substituions during Champions League matches and the rules for the 2021/22 campaign are similar.
Teams during the group stages are allowed to make five changes per game. However, if they make it through to the knockouts, they can make five inside 90 minutes and then an extra sixth sub if the tie goes to extra-time.
To look at the rules in more detail, the Uefa rulebook states: "Each team may use a maximum of three stoppages in play to make substitutions (one additional stoppage in extra-time can be used).
"Substitutions made before the start of the match, during half-time, between the end of normal playing time and extra time, and at half-time during extra time do not reduce the number of stoppages in play that can be used.”
Who is playing in the Champions League this week?
Today’s remaining fixtures began at 8pm BST, including a mouth watering clash between Bayern Munich and Barcelona, while Chelsea host Zenit St Petersburg.
Manchester City will take on RB Leipzig tomorrow night, while AC Milan will travel to Liverpool for the first time since the 2007 Champions League Final.