Boa Morte was a well-documented transfer target for Newcastle, but will out to prove something at St James' Park, according to Clark.
Clark probably won't start against Fulham but may be on the bench, so might not be able to do to stop Boa Morte strutting his stuff.
Clark said of his old Fulham team-mate: "He knows that United wanted him and he will be out to show just what we have missed by not signing him.
"Obviously, I have been watching just how Fulham have been doing this season and I see they are playing a 4-5-1 system with Boa Morte and Steed Malbranque part of their midfield, and I think they are the two players we will have to watch."