(REUTERS) - Manchester United's Ander Herrera is having the most consistent season of his three-year spell in England, but he is still losing sleep because he feels that his position may be threatened if the club strengthen their midfield after the current campaign.

The Spaniard, who joined United from Athletic Bilbao in 2014, has made 42 appearances so far this season, one more than in the whole of last season. He could feature in another 11 games if United make it to the Europa League final.

Herrera set up Marcus Rashford's opening goal in Sunday's 2-0 Premier League win over leaders Chelsea and scored the second, while also man-marking Eden Hazard throughout the game in one of his most eye-catching performances in a United shirt.

"I have to keep working, performing and giving what the manager wants me to give," the 27-year-old told the club's website (www.manutd.com).

"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.

"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."

United host Anderlecht in their Europa League quarter-final second leg on Thursday night.