If you look carefully enough you'll find someone as good if not better than the dude you've lost. Even if it takes a few months you may have to change your formation to only 1 up front with an attacking midfielder.
Hey, I mean I got Talal El Karkouri for £600k thinking heres a nice strong centre half with a good free-kick. what I never knew was that he's a makeshift centre-forward I only had one fit first-team striker out of 6! and that was Ameobi with like 83% fitness, so i benched him and put El Karkouri up front. In 3 games he scored twice and got an assist.
Also if you play some of the youngsters or reserve strikers you might find there better than you think.