The Scottish midfielder collapsed on the field in the second half, without a Reading player around him, and was taken to hospital after lengthy treatment on the pitch.
He was able to stand, with an oxygen mask on, and United confirmed he was conscious when he was taken to hospital by ambulance.
Here’s how we rated United’s players on an evening when the result seemed somewhat secondary...
1. Wes Foderingham 7
Had a nervy moment when a good Reading cross flashed across his goal and he appeared undecided over whether to stay or come for it, but other than that he was relatively untroubled - until a good late save from Carroll's well-hit strike. Hailed with chants of 'League One Wes' by the Blades fans Photo: Simon Bellis
2. Jayden Bogle 8
Frustrated with a couple of overhit crosses in the first half but also showed a couple of nice bits of skill, in a 45 minutes that was overall very low on quality. Had United's best chance of the game up to that point in the second half but his shot was blocked, before opening the scoring with a composed finish at the back stick Photo: Isaac Parkin
3. Chris Basham 7
Couldn't get forward as much as he'd have probably liked, despite the Blades playing against a sole Reading striker for much of the game, but did what he had to do with the minimum of fuss Photo: Darren Staples
4. John Egan 7
A comfortable evening for the Irishman against Reading's sole striker, until they brought Carroll on and went more direct Photo: Simon Bellis