Even though I am not a fan of Erik Ten Hag, it is unfair for Bayer Leverkusen management to sack him after two matches because Bayer Leverkusen key players left the club, and it will be difficult for Erik Ten Hag to get good results immediately at Bayer Leverkusen. Bayer Leverkusen management should have given Erik Ten Hag little time to prove himself and if he fails to do that, that's when they should have taken the decision to sack him.
I think management had other considerations regarding Ten Hag dismissal, not just his failure in the first two matches. Perhaps there were things we don't know about within Leverkusen. The only reason I can think of for this dismissal is that Ten Hag vision and management weren't aligned.
Yes, in this case, you're also right to give Ten Hag time to prove himself. There's nothing wrong with that, but remember, Leverkusen isn't a team with strong finances. If Ten Hag fails this season, Leverkusen will revert to the team it was three years ago. That's why they took swift action.