The 20-time champions of England, who currently sit seventh in the Premier League under the management of David Moyes, confirmed last night that they had agreed a fee, believed to be £37m, with Chelsea for the services of Spanish international Mata.
However, Carragher has claimed that the current United squad is in need of "three or four" players of Mata's ilk to reverse their recent fortunes.
"Now, with United facing a huge fight to get into the top four, they need a new approach. They need to show they are prepared to spend. The clubs who stay at the head of the game invest in the future from positions of strength, which is why the signing of Juan Mata is intriguing," he wrote in the Daily Mail.
"Is this a pointer to the future or simply a break from the norm? Do not think for one moment the £37m paid for Mata will cure all United's ills.
"To get back to where they were under Ferguson, dominating domestically and constantly posing a threat in Europe, they need another three or four of those transfers."