Not sure I'd say he won't improve us, as anyone with pace who can provide good delivery would improve us in my opinion. One its a threat down the left side that will casue full backs worry (not something they've had to worry much about as our wingers on the left have either cut in side or have been slower than paint drying). Beside until we provide real service to Carroll we'll not get the best out of him, and when there is a threat of real service to Carroll, even more space will open up for Suarez to do his magic.
But I get your point he's not exactly what we were looking for. Robben I'd not touch with a 10 foot barge pole. One the injury worries you mention, but I also get the sense he plays for himself not a team, so got to say no thanks. But the quality on the ball, married to speed, and the ability to put a pin point ball into the box is what we need.
That player is not going to be cheap, and if we have the money I still think it should be the priority. However, while a number of names have come up from European leagues, Comoli's interview this week also got me thinking. British, or prem experience players seem to be what they are after, or at least what they put a premium on. So unproven player looking great in the French league but untested against quality defenses, versus a good player who's doing in consistently in the prem?