Wayne Rooney is unsure over Manchester United's decision to sign Casemiro from Real Madrid.

United announced on Friday that they had agreed a deal in principle to sign the Brazilian, subject to a medical, personal terms and other formalities.

Casemiro is set to head to Old Trafford in a deal worth an initial £60 million that could rise to £70m in add-ons, per Sky Sports, and will pen a four-year deal with the club.

The move brings to an end a mammoth nine-year stint at Real Madrid that saw the 30-year-old win the Champions League no less than five times, establishing himself as perhaps the greatest defensive midfielder in the world during that time.

United have been in pursuit of Frenkie de Jong all summer long, but the player's uncertainty over a move to Old Trafford combined with the issue of being owed wages by Barcelona has seen a deal stall. Signing Casemiro could well spell the end of their pursuit of the Dutchman, who had worked with Erik ten Hag at Ajax.

While the Brazilian hasn't actually been linked with a move to United all summer, the move has been largely positively received by fans, due to the fact United have desperately needed a defensive midfielder for several years.

However, club legend and current DC United manager Wayne Rooney has opened up on the moves in an interview with The Times, quoted by the Manchester Evening News, and cast doubt over the deal.

NEW 90m Antony BID (Football Terrace)

Rooney was concerned with the age of player United continue to sign to fix problem areas: "Ideally, they should be signing players in their early to mid-twenties, and Casemiro is similar to Christian Eriksen — someone who has been a good player, but are they going to help the club move forward?

"Frenkie de Jong, Ten Hag’s principal midfield target, would be a better fit."

He added, suggesting that United's approach was scattergun: "The Casemiro deal seems to have come from nowhere, and looks a reactive signing — I’d be surprised if he was a player identified as a priority to bring in when ten Hag arrived.

"It looks like, having been unable to sign De Jong, United just reacted to Casemiro being available."

Pretty strong words from Rooney, who pulled no punches in his assessment of his former club's transfer activities.

MANCHESTER, ENGLAND - FEBRUARY 02: Wayne Rooney of Manchester United celebrates scoring his team's third goal during the Barclays Premier League match between Manchester United and Stoke City at Old Trafford on February 2, 2016 in Manchester, England. (Photo by Alex Livesey/Getty Images)

There is no doubt that a player of Casemiro's quality will improve United, but his concerns over the way in which they signed the Brazilian are admittedly justified, with the deal seemingly coming from nowhere and costing the Red Devils a significant chunk of money.

Consider also that United seem to be making him one of the most handsomely paid midfielders in the world, it's fair to raise questions about how well thought through the signing was.

But, with Ten Hag desperate for new recruits to improve his squad having managed to get just three signings over the line this summer, Casemiro's influence will no doubt be a welcome one when the deal does get done.