The centre-back was replaced by Matthijs de Ligt during the interval at Soldier Field – the only change the boss made before the 70th minute, when the other nine outfield starters were swapped out.
Amorim told journalists after the game that the decision was made on a precautionary basis.
Care is being taken around the England international, who returned to training on the grass at Carrington a little later than other members of the squad due to injury.
“He had an issue during the holidays, during training,” Ruben explained in his post-match press conference. “They were training in the holidays.