This is one of the things that will weaken Manchester City's performance against Real Madrid and Arsenal in the Premier League and in the UEFA Champions League, respectively. Kyle Walker is a good defender, and if he is not in the Manchester City squad that will face Real Madrid and Arsenal, the Manchester City defense will be vulnerable.
Of course, losing Kyle Walker is Man City's weakness, especially since Kyle Walker has been Man City's mainstay defender in the last few seasons. This will affect Man City's performance, especially when they face Arsenal this week, this match will be crucial for both teams in the race for the EPL title.
If you look at the tight competitive situation plus the remaining schedule for the season, Arsenal must win over Man City. The reason is that Arsenal still has to face tough matches such as against Brighton, Aston Villa, Wolverhampton, Chelsea, Tottenham and Man United. So for Arsenal, beating Man City is the best opportunity to stop Man City's title fight. Because, with Man City's experience of winning in the last few seasons, it is very difficult to rely on other teams to beat them.