Evans left the game injured before returning in place of Tom Williams
Harlequins face a Heineken Cup investigation following their quarter-final against Leinster last weekend.
Officials are concerned about the second-half blood substitution of Quins replacement Tom Williams and fly-half Nick Evans' return to the field.
Evans departed injured in the 47th minute, before his replacement Chris Malone was also helped off hurt.
Evans then reappeared after Williams' exit, with television footage appearing to show Williams winking as he left.
Tournament chiefs European Rugby Cup (ERC) said in a statement: "The investigation is being carried out by ERC disciplinary officer Roger O'Connor under the Heineken Cup 2008/09 disciplinary rules.
"[It] will include gathering statements from match officials, the two teams involved and a review of broadcast footage from the game.
"The ERC disciplinary officer will then determine whether any further action is necessary."
Quins lost the match 6-5, with Evans sending a late drop goal attempt wide.