Quins won the competition in 2001 and 2004 and McGahan reckons the Aviva Premiership outfit can seal a third European title when they face Stade Francais in Cardiff on May 20.
McGahan said: "Does that make Harlequins favourites for the final against Stade Francais? They have certainly got a great opportunity if they play with the same spirit and execution as they did against us.
"Full credit to Harlequins. They had a gameplan, were outstanding at the breakdown and in defence and deserved the win.
"We were terribly disappointing in the way we played. We had been showing great passionate and intensity over the last few weeks but that was not there.
"We stood off them and missed too many first-time tackles. And when you do not get phases going you will never put any doubts in the minds of the opposition.