Manchester United manager Ole Gunnar Solskjaer is targeting at least one signing in every area of his squad this summer, sources have told ESPN FC.

Solskjaer said after the chastening Champions League exit to Barcelona on Tuesday night that he is facing a "rebuild" at Old Trafford before United again challenge for top trophies.

The work is set to start at the end of the season with Solskjaer looking to strengthen each area of his team with two new defenders, two midfielders and a forward.

Goalkeeper David De Gea will enter the final year of his deal in the summer but progress over an extension has been slow.

Sources have told ESPN FC the United manager will be backed with significant funds during the summer transfer window and will also be allowed to spend any cash generated by player sales.

Club captain Antonio Valencia is set to lead the exodus after the club decided against triggering a one year extension in the 33-year-old's contract. The Ecuador international has not started a game for United since the 2-0 win over Newcastle on Jan. 2 but, according to sources, is nearing a comeback and could return to the squad for Sunday's trip to Everton.

Ander Herrera is set to leave for Paris Saint-Germain this summer after failing to agree terms on a new contract. Juan Mata is yet to sign a new one-year deal and will become a free agent on June 30 if the stand-off continues.

There are also doubts over the futures of Matteo Darmian, Marcos Rojo, Eric Bailly, Romelu Lukaku and Alexis Sanchez although sources have told ESPN FC there are no plans to sell Paul Pogba despite interest from Real Madrid.

United usually work towards a plan of signing three players during the summer but with more players than usual expected to leave, Solskjaer is set to get special dispensation to bring in more targets.

There is interest in Napoli defender Kalidou Koulibaly, Real Madrid centre-back Raphael Varane, Atletico Madrid midfielder Saul Niguez as well as young English trio Declan Rice, Aaron Wan-Bissaka and Jadon Sancho.

Sources told ESPN FC that after the 3-0 defeat at Camp Nou, Solskjaer told his players he is determined to build a squad capable of reaching Barcelona's level.

"They played very well," said Sanchez, who was applauded onto the pitch on his return to his former club after coming on for the final 10 minutes following his return from a hamstring injury.

"We had very clear chances, but this is football, this is the Champions League, if you get something wrong or fail then the other team gets on top of you. That's what Barcelona do.

"When I came on to the pitch I heard the applause and the truth is I am very happy about this.

"It was a nice moment for me. With the other players we won good things here and I am very happy to return from my injury and I am also very happy with the applause from the crowd."