The left-back was brought off with a hamstring problem after just 22 minutes against Manchester United on Monday and is now doubtful for the club's semi-final tie at Wembley
The left-back was replaced by Ryan Bertrand after just 22 minutes in Chelsea's FA Cup quarter-final win over Manchester United on Monday afternoon and is now a doubt for the club's semi-final tie against Manchester City on April 14.
"This kind of injury he'll [Ashley] be out for two weeks," Benitez told reporters.
Chelsea centre-back Gary Cahill also missed Monday's tie at Stamford Bridge due to a knee injury and Benitez also confirmed the England international is likely to be sidelined for around two weeks.
Chelsea currently sit fourth in the Premier League, two points ahead of fifth-placed Arsenal, with eight games remaining, and Benitez insists securing Champions League football is the club's main objective.
"The FA Cup was an important trophy. Now we are going to focus on the league, it's nearly done, but of course, today is a really sad day for Manchester United," Benitez said.
"The priority for us is to finish in the top four but in a top side you have to keep in every competition and try to win it.
"The race for the top four will be up until the end of the season but we are in the top four with a game in hand, we are in the semi-finals of the FA Cup and the Europa League quarter-finals, so it is a great season so far."