I understand what you’re saying, but any transfer is a gamble. Besides I seem to remember Grosicki and Hernandez hitting the ground running.
Grosicki didn't score his first goal until we were in the Championship, he's a bad example to use. Hernandez worked well enough, but we were in the PL with a far stronger squad around him. And even then I think he only scored 3-4 goals in between lengthy spells out injured.