The Magpies look destined for the Championship this season.
The Magpies' Premier League survival grows more uncertain by the week as Rafael Benitez's side sit 19th in the table, six points behind 17th-placed Norwich City after losing 3-1 at Southampton on Saturday afternoon.
United - then under Steve McClaren, who was fired last month - drafted in midfielders Henri Saivet, Jonjo Shelvey and Andros Townsend for the guts of £30 million in January, along with the loan signing of striker Seydou Doumbia.
But the legendary Newcastle striker struggles to understand why the club did not sign defensive cover - particularly a centre-half - after suggesting that Chancel Mbemba has not been up to scratch in recent weeks.
“It is also staggering that a centre-half was not signed in January given how they have struggled defensively all season — conceding 61 goals in 32 league matches," a frustrated Shearer told The Sun.
“Even last summer, only DR Congo star Chancel Mbemba came in and, having played OK early on, he has also looked out of sorts in recent games."
Newcastle have something of a lifeline in the form of a game in hand, but even that is a visit of Manchester City, who are hunting Champions League qualification.
It certainly appears as if the club are heading down for what would be its second relegation in seven years. They host Swansea City on Tyneside this Saturday.