Manchester United boss Ole Gunnar Solskjaer has challenged his players they deserve to stay ahead of a planned summer overhaul.
The London Evening Standard says Solskjaer is targeting some of the best emerging English talent in Jadon Sancho, Callum Hudson-Odoi, Declan Rice and Aaron Wan-Bissaka.
Everton's 29-year-old midfielder Idrissa Gueye is seen as a potential replacement for Herrera.
Napoli centre-back Kalidou Koulibaly, 27, is also a long-term target.
But Solskjaer wants reactions from players already at his disposal, too, with Scott McTominay and Fred producing some encouraging performances in recent weeks to suggest they could play more prominent roles next season.
“That's what we expect from all the players," he added. “When you win 5-0, or when you're 3-0 up, or when you win five games on the bounce, it's easy. It's easy to be a footballer.
“But it's not always easy to be a footballer. You get through tougher times. You'll lose games and your character is revealed."