It is alleged that Mackie, making his return to the first team after a month in the reserves, was waiting for Asaba in the tunnel. He then got involved in a fracas with the Blades frontman.
It is reported that there were then some afters when the players came back out onto the field for their warmdown just minutes later.
The 26-year-old centre-back was understood to have been upset about an elbow that had been swung at him by Asaba.
Sheffield United boss Neil Warnock explained to the club's official site exactly what he saw happen. During the game it is reported that Mackie 'angrily pointed his finger' at Asaba - but that wasn't the end of the incident.
"Mackie waited for Carl in the tunnel afterwards and there was a bit of a fracas," explained Warnock.
"Some racial remarks were made and directed at Carl. When the players went to warm down, there were some racial remarks directed at Carl which we are not accepting. John Salako and some other Reading players heard the comments.
"Carl Asaba had decided not to take the matter further, but as a club we will be writing to the FA and PFA because we will not accept this.
"It is up to the FA whether they want to take the matter any further, but they are trying to stamp out this kind of behaviour in football.
"We didn't mention it on Saturday, because we didn't want the incident to detract from our excellent result, but our players were upset after the game."
If the speculation suggesting that Mackie racially abused the black former Reading striker is found to be true he could find himself in very serious trouble as both the FA and PFA attempt to stamp racism out of football.