The Red Devils boss has faced an uncertain December and January following one win in eight Premier League games since a 2-1 victory over Watford in late November.
The 20-time English champions have been linked with axed Chelsea manager Jose Mourinho and Bayern Munich boss Pep Guardiola in recent weeks.
The Red Devils twice squandered a lead in a 3-3 draw with Newcastle United at St James’s Park on Tuesday night after Paul Dummett’s 89th-minute equaliser.
Former Portsmouth and Tottenham Hotspur manager Redknapp believes there are a lack of available managerial alternatives if United were to sack Van Gaal.
Redknapp told BT Sport: “When you talking about sacking him, the people who they could replace him with aren’t available.”
The Red Devils make the trip to Anfield to face bitter rivals Liverpool in the Premier League in Sunday’s early kick-off.
Van Gaal’s men are in sixth place but five points behind fourth-placed Tottenham after their 4-1 win against Sunderland at White Hart Lane.
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