Johnson out for a month

Johnson will be out for at least a month (GETTY IMAGES)

Liverpool defender Glen Johnson has been ruled out for at least a month with a knee injury.

The England right-back sustained a tear of the medial ligament in his right knee during Tuesday's 1-0 victory at Aston Villa.

No definitive timescale will be put on his recovery until Johnson visits another specialist to assess the extent of the damage.

"Johnson has a problem, so he'll see a specialist and then we'll talk but we know he'll be out for at least one month," said Liverpool manager Rafael Benitez.

"But we are waiting for another opinion and then we'll know how long."

Right-back has become a troublesome position for Benitez as Swiss international Philipp Degen has struggled to settle after injury wrecked his first season after signing in the summer of 2008.

The 26-year-old did not make his Barclays Premier League debut until this September and has been unconvincing in the first team, getting sent off in the 3-1 defeat to Fulham at the end of October.

Benitez's likely option is to move Jamie Carragher from central defence to fill a role he is not entirely comfortable with.

He could also give Stephen Darby, 21, an extended run in the side while 19-year-old Martin Kelly - who made his first start in the Champions League defeat at home to Lyon in October - is currently sidelined with injury.

If the Liverpool boss should decide to dip into the transfer market it is unlikely he would be looking for anything other than a short-term loan deal.

On the positive front, Benitez is hopeful of having Albert Riera back from a hamstring problem within the next few days.

"Riera will start training tomorrow, so hopefully in the next week he can be available," said Benitez.

"Kelly and Nabil El-Zhar still need maybe one or two weeks."