The left-back was replaced by Ryan Bertrand after just 22 minutes of Chelsea's FA Cup quarter-final win over Manchester United on Monday and is now a doubt for the club's semi-final against Manchester City on April 14.

"This kind of injury he'll be out for two weeks," Benitez told reporters.

Chelsea centre-back Gary Cahill also missed the 1-0 victory at Stamford Bridge due to a knee injury.

According to Benitez, the England international is also likely to be sidelined for two weeks.

Having suffered a surprise 2-1 loss to Southampton on Saturday, the Spaniard called for a sense of perspective when assessing his team's results.

"It is interesting because before the international break we played against United, we played against Steaua (Bucharest) and we played against West Ham, and we were the best in the world," he said.

"We lost against Southampton and we were the worst in the world and today we have won and again we are the best.

"I think that you have to have balance, and we knew it could be like this, we knew that we could win and we could lose but we have to carry on.

"The priority for us is to finish in the top four, we know. But in a top side you have to approach every competition trying to win."

United manager Alex Ferguson said his team struggled to get back in contention following the goal from Ba, a player he identified as a major threat going into the game.

"After that we lacked composure to win the game. We had a lot of possession but didn't really make use of it," Ferguson told ITV.

"We spoke about Ba and the experience we had in Newcastle (a) couple of years back. He scored exactly the same type of goal and we always had to be aware of the type of execution he has with his shots.

"We were caught napping to be honest with you and right at the start of the second half, it wasn't a good start for us."