"I am pleased to be bringing in an experienced player who has been at a much higher level in his career," Boothroyd told the club's official website.
"David will bolster our central midfield area for the games to come."
Manchester United's teenage striker Marcus Rashford scored on his senior England debut against Australia at the Stadium of Light.
Real Madrid take on neighbours Atletico in the Champions League final on Saturday night.
Real Madrid face neighbours Atletico in the Champions League final on Saturday looking to secure their 11th European crown.
Manchester United sacked Louis van Gaal as manager this week, with the team's style of play under the Dutchman reportedly key to the decision.
Jose Mourinho has been named as Manchester United's new manager - an appointment that unites a man whose handling of
The rivalry between Jose Mourinho and Pep Guardiola will be renewed next season, with the pair taking charge of the