The Red Devils are sweating on the fitness of captain Vincent Kompany and Thomas Vermaelen, while the Stars and Stripes are monitoring the progress of Jozy Altidore
Belgium boss Marc Wilmots and USA counterpart Jurgen Klinsmann both have injury concerns ahead of Tuesday's last 16 showdown at Arena Fonte Nova.
The Red Devils are looking particularly threadbare at the back, with Vincent Kompany struggling with a groin problem and Thomas Vermaelen yet to recover from a hamstring strain.
Anthony Vanden Borre’s tournament has also been ended by a fractured fibia sustained in the win over South Korea but Toby Alderweireld was poised to slot back into the defence at right-back anyway, having been rested for his country's final game in Group H.
Elsewhere, Steven Defour is suspended, so Axel Witsel will definitely return in midfield but doubt remains over who will lead the Belgian attack at Arena Fonte Nova.
Wilmots can either persist with the goalless Romelu Lukaku up front or take a chance on Divock Origi, who has come off the bench in his country's three games to date, scoring a late winner in the 1-0 defeat of Russia.
However, there is also the option of asking Edin Hazard or Kevin Mirallas to serve as a 'false nine', thus creating more space for another of the Red Devils' many attacking midfielders.
The Americans have their own selection issues up front. Team USA lost Jozy Altidore to a hamstring injury just 23 minutes into their tournament opener against Ghana, but he returned to training last week.
Klinsmann, therefore, must decide whether to recall the Sunderland striker and restore Clint Dempsey to a supporting role - or continue with the latter up front on his own.
Elsewhere, Omar Gonzalez is expected to retain his spot alongside Matt Besler at the back after turning in a fine performance in place of Geoff Cameron in the game against Germany.
However, Alejandro Bedoya could return on the right-hand side of midfield after being cleared of concussion, with Graham Zusi switching to the opposite flank and Brad Davis dropping to the bench.