First of all, I hope all is well with you bro..
We signed a 26 year old Jaap Stam for a record fee from PSV in 1998. And he then went and played in the world cup in 98, and had a poor tournament for his standards, before making his debut for us in the Charity Shield where Nicolas Anelka ripped him a new one in a 3-0 win for Arsenal. The general consensus was that we'd signed a dud, but after a settling in period, Stam went on to win the treble with us that same season.
Nerves do get the better of players at times and it takes a bedding in period for some to settle into high pressure environments.
That doesn't mean Diogo Costa is the right choice but rather we shouldn't look too much into what happened at the world cup imo.
For me, we should look to sign a well rounded keeper. And Diogo Costa along with Anatoliy Trubin, Gregor Kobel and Maarten Vandervoordt should be among the targets imo. I'm also hopeful a new owner will be in place before the summer window.