"Their focus on the Premier League, I don't know... recent results suggest maybe the focus has diminished," Hughes told a news conference on Thursday.
Chelsea are currently fifth in the league with 57 points, 10 points behind Tottenham Hotspur, who are a place above and occupy the final Champions League qualifying spot.
"Their priorities prior to the season involved Premier League and Champions League titles, clearly those have fallen by the wayside," Hughes added.
"If they have a poor period, more often than not top teams bounce back so we're not working towards the weekend thinking that Chelsea are going to be under par.
"We have to prepare as if Chelsea are going to be at their best, with their best players available."
Saturday's clash between the two teams will serve as a dress rehearsal for this month's FA Cup semi-final but Hughes said he was not looking too far ahead.
Southampton are 18th in the league with 28 points, three points adrift of the safety zone.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Toby Davis)
