After months of speculation, Manchester United have finally confirmed that Ander Herrera is set to leave United by the end of the season. Herrera left a heartfelt message for United fans on social media, which says a lot about him as a player and as a person.I could never express enough gratitude 🔴 #MUFC https://t.co/wwqNHFnXE1— Ander Herrera (@AnderHerrera) May 11, 2019He said:A club with thousands of fans that respect and remember all the players who give everything.I felt special when I heard my name chanted. I felt proud when the fans decided I was part of this incredible history.Every time I represented this club, in every game, in wins and losses, even when I couldn't help from the grass I understood what this club means.This season, United have been extremely poor on the pitch and Solskjaer has some major rebuilding to do this summer. In such a scenario, a player of Ander Herrera's calibre would have been priceless for the Norwegian. Unfortunately, United have failed to keep him at the club and here are 5 reasons why the Red Devils were wrong to let Ander Herrera leave.#5 His replacement will be costlyCardiff City v Manchester United - Premier LeagueTo start rebuilding his team in the summer, Ole Gunnar Solskjaer will definitely expect a like-for-like replacement for Ander Herrera. Because of the nature of the position and United's recent reputation in the transfer market, the move will neither be cheap, nor easy for the management.In the current scenario, there are few players with the qualities of Ander Herrera available for transfer. There was initial belief that Scot McTominay was the right guy to fill the Spaniard's shoes at United. However, recent games have hardly suggested that the Scottish midfielder would be able to fill Herrera's boots on a regular basis. As such, it is clear that United will have to invest in the market for a midfield warrior this summer.If reports are to be believed, Manchester United have identified Portuguese Midfielder Bruno Fernandes as the ideal replacement for Herrera. However, the Portuguese is also believed to be in City's radar and even if Pep Guardiola's side have cooled their interest of late, Fernandes would still cost upwards of £50m. That is a lot of money for the 24-year-old who is unproven in a big league.