We won't know until they get the chance. Based on what you said, which was something like 'in that case we will fail', here are three examples of newly appointed managers who have achieved great success with major clubs in their first appointments.
Iraola, as one example, came through the CFG and has proven himself very well with Bournemouth.
I have read that Gunodgan has done incredible work with the U16 team and impressed on the FIFA coaching courses. Regardless, I am sure that we already know who Pep's successor will be and we will have been scouting the market for several years.