Ah injuries, seemingly the bane of Liverpool's existence. Constant injuries to Fernando Torres helped kill our hope of the Premier League title back in 08/09, while injuries to him, Steven Gerrard, and Alberto Aquilani helped to derail our 09/10 campaign, which ended with us finishing 7th. Last season saw Steven Gerrard undergo season-ending groin surgery, while Martin Kelly tore a hamstring and Fabio Aurelio continued to be made of glass.
This year, a deeper squad and a lack of European competition should help to alleviate some injury concerns. Of course, many players in our squad are also frequently playing in international competitions, which add many (generally) unnecessary games. Liverpool may not be ready to mount a real title challenge just yet, but finishing in the top 4 to qualify for the 12/13 Champions League is the goal, and any injuries to key members of the squad will be a huge deterrent to this.
As arguably the best goalkeeper in the league, Pepe Reina has been a constant fixture for Liverpool since his arrival from Villareal in 2005. At 28, he is just entering his peak years as a goalkeeper (though as Van Der Sar showed last year, a goalkeeper can still perform brilliantly even when out of his normal "peak range" of roughly 28-32), and will be a key component to Liverpool's squad. However, with the signing of the experienced Brazilian international Doni, if Reina was to miss a short period of games due to injury, it would not affect the squad as much as if Reina had been injured in previous years with less experienced back-ups.
In attack, Carroll joined Liverpool injured last season, and was never able to reach peak fitness with the team. While he may become a key piece of the team this year, his absences last year showed that the team could perform without him. Luis Suarez, meanwhile, instantly became a key piece of the team. Without him, Liverpool would be much less creative in the attack, and a much less effective team. With any luck, he will avoid major injuries this year.
With our midfield as crowded as it is, it may seem odd that an injury there would affect our season. However, as our only "true" defensive midfielder, Lucas Leiva proved invaluable to the squad last year. In pre-season this year, Liverpool conceded 3 goals in every game except against Valencia, when Lucas was able to play the second half. His presence in the midfield will be very helpful, and if he does have an injury, it seems that Jay Spearing (or a much more disciplined Gerrard) will be his only replacement.
Liverpool's defense seems to have suffered the most injuries in recent memory, with Johnson, Aurelio, and Agger are missing significant periods of time. Even last year's "iron man" Martin Skrtel will be starting this season on the bench due to a calf injury. With a deal for Jose Enrique seemingly close, Aurelio's injuries will hopefully not be as big a concern. A deal for Scott Dann also seems to be in the works, but not as close as the Enrique deal. With no idea how long Skrtel will be out, the only back-up to Carragher and Agger is Kyrgiakos. And many fans see Agger as our only CB worthy of the first team, with Carragher only getting older and slower. Agger, meanwhile, is almost always brilliant when he plays, and makes our defense much better.
So in my mind, we have a triumvirate of Suarez, Lucas, and Agger as players we can't afford to lose to injury. Agger will, sadly, inevitably miss at least 12 League games, but if Suarez and Lucas do as well, our season may be in serious trouble. On the other hand, with the 3 can combine for at least 90 League games (and ideally 100), the top 4 will be almost a certainty.