Yes, they should have bout at least 2 very good defenders. I'll bet that they tried for some but didn't get them, for a number of reasons.
But.
Falcao is a superb player, bangs goals in for fun and is commonly known as world class. Like LVG said, when a player like Falcao comes available, you don't pass it up.
Time will tell if he is a waste of money. I sincerely doubt it.