The midfielder is still complaining of discomfort from a blow which forced him to come off at half-time against the Champions League winners on Sunday.
The Red Devils' medical staff were hoping the Spaniard's complaint would settle down, but there has been little sign of improvement.
Herrera has been ordered to take it easy and is rated a doubt for the final tour game against Barcelona on Wednesday in Washington DC.
United boss Jose Mourinho is refusing to take any risk with Herrera, named the club's Player of the Year last season and who was recently handed the vice-captaincy due to his ever-growing influence in the team.
The general consensus is the player has suffered some heavy bruising and United have now asked for an X-ray to be sure.