2 weeks he's been in charge, he was already on holiday during that point so lets say a week it's been a week since he was at Goodison and Finch Farm.
He's been/is back since/on Saturday, depending who you believe and pre-season starts Monday. We won't be signing last minute deals as the window is yet to open/opened 3/4 days before Koeman actually took charge.
Once he's evaluated the squad, deals will be done. It's easy to sit on paper and say "Well, Oviedo played against us, let's keep him" because if it comes to next week and for example Oviedo doesn't perform well enough, he'll get shut. Simple as.
He's not had the time to actually to sit and look at the squad in action and shape it to his likings. It takes time.