Shaw, 23, revealed this week that he could have lost his right leg after suffering fractures of the tibia and fibula during United's Champions League tie with PSV Eindhoven in September 2015.
Despite returning just under a year later, Shaw had struggled for form until the start of this season when four impressive performances for the Red Devils earned him an England recall.
He repaid the faith shown in him by England manager Gareth Southgate with an excellent cross for Marcus Rashford to open the scoring against Spain.
However, his night was ended prematurely when he collided with Carvajal in the air just after half-time, with Shaw's head crashing off the turf as he fell to the ground.
Southgate and United boss Jose Mourinho, who was in attendance at Wembley, now face an anxious wait to see how long Shaw could be sidelined.
At the very least, the former Southampton defender looks set to miss England's friendly against Switzerland on Tuesday.