Lockdown is not only rough for wear, but his hook got melted down by Meltdown's acid. It happened off-camera even though I originally planned to show it, well, actually happening. Hey, this comic got a lot longer then I originally envisioned! I just want it to end (and it will, got a three or so more pages left).
AWWW MELTDOWN YOU DID PRACTICALLY NOTHING. But it does raise an interesting dilemma as now the events of "Black Friday" is completely different than what canon showed.
Lockdown is more upset over Prowl dying than his arm. It's an indication that he cares for him, if not a little. Besides, like you said, he can fix/replace it.