Hendo for RWB!
Really depends on what formation you're playing, but if you use Wing Backs who like to overlap and get involved in attacking, I'd recommend re-training Jordan Henderson to cover for Johnson.
Every Liverpool save i've played, Johnson always ends up getting injured for a few months... believe it or not, Henderson fits in there perfectly on offense, and isn't too bad defensively either. He started 35 times for me this save (1st year) across all competitions, most of which were from the RWB position. Johnson had to fill in for Enrique at LWB, who was out for about two months, which opened up the right side spot for Kelly and Henderson. Henderson won the starting job, and ended up scoring 3 goals and creating 9 assists with a 84% passing rating and 5.16 tackles per game, rounding a decent 7.16 avg rating all year.
Not bad for a natural midfielder who is already on your team.