Something clubs always need to look at during the transfer window isn't just improving their best eleven, but also at improving their depth. You can't always put out your first-choice side in every match, so keeping the dropoff to the second unit as small as possible is a priority, especially for a club potentially facing continental competition next season.
Bringing in players expressly to be a backup isn't always easy. Guys want to get playing time, to make a big name for themselves. However, a club of Liverpool's stature that's enjoying the success they're currently seeing can pull it off with promises of future success, especially if they can stay on track to make next season's Champions League. Being able to take a reliable player from a smaller club and offer higher wages for less work for a more prestigious club doesn't hurt, either.
Sebastian Larsson, Right Midfielder, Sunderland
Being a quality backup generally involves versatility, and Larsson has it in spades. Primarily a right-sided midfielder, the Swedish international can also play along the left and as a box-to-box central mid if needed. Should Brendan Rodgers wish to continue experimenting with the 3-5-2 in the future, Larsson would also function as a good option at wing back.
While taking a player who's normally a starter (or at least frequent rotational option) and turning him in to a backup is never easy, being able to offer him a way out of the EPL's last-place club and drop him in to a team currently in position to contend for the title could make enticing him to join easier. Larsson, or a player much like him, would be an excellent addition for Liverpool to consider.
Wayne Routledge, Left Winger, Swansea City
Liverpool's need for improved depth on the wings isn't really a secret. While Routledge isn't a star by any means, he's a solid player with a good cross and technical skills out wide. He's better out on the left side, but the dropoff with him on the right is small, which is a boon.
He's been a regular in Swansea's side over the past two seasons despite more skilled wingers like Nathan Dyer and Pablo Hernandez being in the squad because he's been a dependable player who doesn't suffer from prolonged stretches of poor form. Add that to a strong work rate and pressing ability, and he'd be a solid option to add to the Anfield roster.
Anthony Vanden Borre, Right Back/Right Mid, Anderlecht
The Belgian international is a wonderfully versatile player, capable of playing up and down the right flank as well as a little bit of central or defensive mid if needed. He'd be inexpensive in terms of both wage and transfer fee, but would easily help Liverpool's depth situation. The Belgian hasn't been in his national team for awhile, but had earned 22 caps with the senior side, and showing an ability to perform in the EPL would only help his cause to return.
Vadis Odjidja-Ofoe, Central Midfield, Club Brugge
Odjidja has been trying to get to the EPL for quite awhile now, most notably with multiple efforts to get from Belgium to Everton that have fallen just short. The Belgian is a dynamic midfielder, capable of filling several different roles, from supporting a striker to shielding the center backs. He's best as a box-to-box support midfielder, but his role versatility is always good to have off the bench should an injury or need for a tactical shift occur.
Other Players To Consider: Ludovic Obraniak, Attacking Mid, Girondins Bordeaux (France); Vasilis Torosidis, Right Back, AS Roma (Italy); Olcan Adin, Left Winger, Trabzonspor (Turkey); William Kvist, Central Midfield, VfB Stuttgart (Germany)