Guthrie, sent off near the end of Hull's 2-1 win at St James' Park, has already been hit with a three-match ban for violent conduct.
Although the FA have increased suspensions in the past, they have never added extra matches because of the seriousness of an offence.
But an FA spokesman said: "We are looking at the incident and have the right to alter the suspension if necessary."
Guthrie issued an apology yesterday and was trying to contact Fagan to say sorry personally but that was being blocked by Hull staff.
He said: "I was very upset to hear the news concerning Craig. The club have been in touch with Hull on my behalf because I want to speak to Craig in person to apologise and wish him a speedy recovery. I hope to do that as soon as possible.
"It was a challenge that came purely out of frustration and anybody who knows me will accept it was out of character on my part."