Heaven was seen writhing in pain after his foot fell under the body of Patson Daka during United's 3-0 win at Leicester City in the Premier League on Sunday. He had to be carried off on a stretcher amid fears he might miss the rest of the season.
Heaven's injury leaves United with just two three fit experienced defenders in the squad: Victor Lindelof, Matthijs de Ligt and Noussair Mazraoui. Amorim gave Harry Amass - who was celebrating his 18th birthday - his senior debut when he brought him on in the second half. Heaven's injury worsened a dire situation for United in defence, with Harry Maguire, Leny Yoro and Luke Shaw already missing.
It is set to be a huge month for United, who face Lyon in the Europa League quarter-finals as well as hosting arch-rivals Manchester City in the Premier League.