"We picked the strongest team that we could," Perez told www.saintsfc.co.uk. "Because there were players that needed to recover and some had illness or lack of freshness.
Manager Mauricio Pochettino brought Victor Wanyama and James Ward-Prowse into midfield and Guly do Prado into the attack, while Kelvin Davis replaced Artur Boruc in goal. Joos Hooiveld and Nathaniel Clyne came in for Jose Fonte, the scorer against Hull, and Calum Chambers in defence.
The side looked lacklustre for long periods and Rickie Lambert, who did feature in mid-week, squandered the visitors' best chance to equalise when he scooped the ball over from virtually under the crossbar late in the match.
Craig Gardner had put Sunderland ahead in the 49th minute, letting fly from 25 metres.
"We decided to pick the best XI after a hard game at Hull on Tuesday night," Perez said.
"Our team is strong and the core of the squad is inside the dressing room with any player available to play.
"The scoreline isn't because who we played today.
"We are a little bit sad to lose this opportunity to go forward in the FA Cup, but that was not because of the players."
Southampton's next league match is away to West Ham at Upton Park on February 22.
(Reporting By Mark Pangallo; editing by Clare Lovell)
