Sheffield's low probability at home is not a complete guarantee that Arsenal can win the match. We don't know what will happen even though we believe Arsenal can win the match, but unexpected things are surprising in the Premier League.
Nothing is guaranteed in football, it’s XI vs XI but let’s be honest here. Arsenal are probably the most in form team in the PL. They are battering everybody, scoring lots of goals in recent weeks.
I will be very surprised if Sheffield United get anything other than comfortably beaten tonight.