It looks as though one of the assistants has taken ill at half time at Brammall Lane leaving Ben Toner to run the line for the second half
From the BBC site:
'Is there a qualified referee in the house?'
There was one moment of drama for the officials as United went through, with assistant referee Daniel Leach having to be substituted at half-time because of illness.
An appeal was put out over the public address system asking for any qualified match officials in the stadium to make themselves known to a steward.
Tyler Machin, watching from the stand, answered the call, making it a busy day for the teenager as he had refereed a local league match in Worksop in the morning.
Machin took over as fourth official, with Ben Toner stepping up from that role to run the line in the second half.