Both Manchester clubs passed on the chance to acquire Vidal after being offered the Chile international before Barca made their move, per Jose Alvarez Haya of Spanish football television program El Chiringuito de Jugones:
Barca were the beneficiaries, agreeing a three-year deal with one of the few complete midfielders in the modern game, per a statement on the club's official website.
Concerns about Vidal's injury history make sense, but it's still surprising City would pass on Vidal. Pep Guardiola is in charge in the blue half of Manchester and signed Vidal for Bayern from Juventus back in 2015.
Age also seems like a strange reason for United to turn down a deal for Vidal since Jose Mourinho is at the helm at Old Trafford. Vidal is 31, but Mourinho has a history of placing his faith in experienced players over developing youth.
At his best, Vidal is a tireless workhorse who makes box-to-box runs. He is aggressive enough to act as a destroyer and break up play while also possessing the technique and vision to both create and score goals.
Given his attributes, Vidal would have been an asset at either City or United. The former were denied the chance to strengthen in midfield when Premier League rivals Chelsea beat them to the signature of pass-master Jorginho, who moved to England from Napoli in a deal worth £50 million.
Not landing Jorginho has left City asking for a work permit so the 20-year-old Douglas Luiz can be part of the first-team squad this season, per Rhys Turrell of the Daily Star. Adding a proven commodity in Vidal would have provided something different in a midfield dominated by schemers David Silva and Kevin De Bruyne.
United could also have used Vidal despite having signed Fred from Shakhtar Donetsk earlier during this summer's transfer window. Since then, Nemanja Matic has been ruled out for the start of the new season after undergoing surgery, per BBC Sport.
The bigger problem concerns the future of Paul Pogba, who continues to be linked with a move away. Pogba's agent, Mino Raiola, is set to attempt to engineer a transfer to Barcelona for the Juventus and Paris Saint-Germain target, according to the Daily Star's Paul Hetherington.
Vidal might be considered to be on the downside of his career, but the fact a club of Barcelona's stature still came calling speaks volumes about his talent. City and United may regret turning down this transfer.