Police Scotland confirmed MacKenzie was treated at the scene and will require further medical assessment.
They have also appealed for information and asked that anyone who witnessed the incident or has footage to contact them.
Match Commander, Superintendent Ray Birnie, said: «This was a mindless act which has seriously injured the player, resulting in him needing medical treatment.
«This type of behaviour will not be tolerated and I’d appeal to all fans to help us trace the person responsible.
«If you were in the Fair Play Stand and witnessed what happened, please do the right thing and come forward. I’d also urge anyone who has mobile phone footage to review it and bring anything of importance to our attention.»
Goodwin, who has experienced similar incidents in on previous occasions in which he managed United against former club Aberdeen, labelled the scenes «a disgrace».
«I deliberately took myself off the pitch as quickly as I could because we’ve had previous with the Aberdeen supporters,» he said.
«I’m really disappointed to hear young Jack has been hit by an object thrown by his own supporters. He’s inside having stitches now, it’s an absolute disgrace.
«It’s probably taken something as serious as this to happen for the authorities to finally step in and do something about it to identify these idiots who let down a really good club.»
United are yet to comment on the incident, but the SPFL critised the «moronic behaviour».
«It’s enormously disappointing and frustrating that one of the most exciting games in the season has been marred by such moronic behaviour,» a statement read.
«We await the match delegate report but will work closely with both clubs and Police Scotland to help identify the perpetrator and fully expect that appropriately severe footballing and judicial sanctions will be imposed.»
