Frisk was hit by the missile - believed to be a cigarette lighter or coin - as he left the pitch at half-time just moments after he had sent off Roma defender Philippe Mexes for kicking out at Kiev striker Maris Verpakovskis.
Frisk who required medical treatment for a cut on his forehead abandoned the contest at the interval with Kiev leading 1-0 thanks to a Goran Gavrancic free-kick strike.
The UEFA disciplinary unit today began their inquiry but any announcement is not expected before next week.
"This is a very serious matter. The security and safety of the supporters players and officials is our top priority so we need to look closely at the events UEFA communications director William Gaillard told PA International.
"We will be looking into the incident but it is really premature to be talking about punishments.
"We will be looking at the referee's report of the match and the report from the match delegates which is the normal procedure."
It is not the first time that matches at the Stadio Olimpico have attracted serious crowd trouble.
As recently as last season Roma were fined and handed a ban from playing home games at the stadium after rioting at the derby clash with Lazio left over 150 people injured.
However Gaillard is not prepared to jump to any conclusions.
"There was no other apparent crowd trouble and everyone waited peacefully in the minutes following the incident he added.
"It is always necessary to look at the mechanics of the incident because it is always possible the person who did it might not be mentally sound.
"Remember what happened in the marathon at the Olympics for example (when race leader Vanderlei de Lima was attacked by a spectator and ended up finishing third)."
Gaillard also confirmed that UEFA would look at why Frisk had taken the decision to halt the game rather than place the fourth official in charge.
Frisk felt his replacement was too inexperienced for the circumstances and although the referee has the authority to call off a game Gaillard admitted the Swede had not followed the "usual procedure".
He added: "We are going to look into that and the report from the match delegates is going to be important.
"The rule clearly calls for the fourth official to step in if the referee cannot continue.
"That is the usual procedure and we don't know why Anders Frisk did this.
"Maybe he thought he (the fourth official) was not experienced enough but we have to wait to see his report because he might have another reason.
"Of course there is also the possibility that the incident left him (Frisk) quite traumatised."