The Spaniard joined United from Athletic Bilbao in 2014 and has enjoyed the finest season of his three-year spell in Manchester under Jose Mourinho this year.
Herrera has made 42 appearances so far this season and was particularly impressive in United’s 2-0 win against Chelsea last weekend, setting up Marcus Rashford's opening goal and scoring the second, while man-marking Eden Hazard throughout the game.
But that hasn’t stopped the 27-year-old fearing for his position at Old Trafford.
“I have to keep working, performing and giving what the manager wants me to give," Herrera told the club's website on Wednesday.
“We have great players and Manchester United has the capacity to sign big players, so I cannot sleep.”
United broke the world transfer record when they signed Paul Pogba from Juventus in August and manager Jose Mourinho is likely to continue revamping the squad when the transfer window opens in July.
Herrera has emerged as a key player under Mourinho, starting 23 of the United's 31 league fixtures so far this season compared with last season's 17 starts under Louis van Gaal.
READ MORE: PSG ‘to offer £46m’ for Alexis Sanchez
"I am very thankful to him (Mourinho). This season has been the most regular one for me at the club and that is with Mourinho on the bench," he said.
"I am very happy. He gave me, from the first day, a lot of confidence and he told me I was going to be an important player for him."
(Additional reporting by Reuters)