The 25-year-old Argentina World Cup winner was omitted from the squad for the a 7-0 win over League One Port Vale in the FA Cup quarter-finals and last week’s 3-0 Premier League defeat by second-placed Manchester City at Stamford Bridge.
Fernandez, who had been wearing the captain’s armband in the absence of the injured Reece James, had previously said he would assess his future after the World Cup and expressed a desire to live in Madrid, comments for which he later apologised.
"Enzo has been with the group and has been training very, very well," Rosenior told reporters on Thursday. "So, it’s business as usual in terms of selection for the game.
"In terms of his training, his application to training, Enzo has been fantastic as well."
Rosenior also provided updates on defender Trevoh Chalobah, who has not played since the second leg of Chelsea’s Champions League defeat by Paris Saint-Germain, and on James, who has been sidelined since the loss to Newcastle United also in March.
"Trevoh trained today but not quite fully; it was a modified training," Rosenior said. "We'll make a decision on him, but he's very, very close. Reece is a little bit further away."
Chelsea are sixth on 48 points, four behind Liverpool who are in the Champions League spots, having won only one of their last five Premier League matches. Victory over third-placed United would help close the gap in the race for the top five.
"You focus on what you can control, which is our performance," Rosenior said. "You make sure the attitude, the energy and the application to the game is right. I expect a good performance on Saturday."