Section 15.13 states:
One player from the above quota of three who has played for another competing club in the current season may exceptionally be registered by 1st February 2006, provided that his new club is taking part in a UEFA club competition other than the one in which his former club is participating or has participated in the current season.
So, to the letter of the law Rochemback is inelligible, not because he played for Sporting in the Champions League, but because he played for Sporting in the UEFA Cup first round.
However, since Sporting were eliminated from the UEFA Cup before the group stage, Boro have won an appeal on 'exceptional circumstances' and the Brazilian will be eligible for selection in the round of 32 against Stuttgart.