The Blues, meanwhile, have less to play for but could still leapfrog Connacht into eighth spot and while Lions captain Sam Warburton has not been selected, winger Alex Cuthbert has.
And Diack, who along with Chris Henry will be making his 100th appearance for the province, is adamant there will be no complacency in the Ulster side.
"Knowing that a win would mean a home semi-final is a huge motivation," he told the Belfast Telegraph. "But we have to win first.
"I think it will be tough for any side to come to Ravenhill and beat us in front of a home crowd so that's a huge factor for us, too. But we can't start thinking about semi-finals or anything else until the end of 80 minutes on Friday night.
"It's going to be a big night for me. To be getting my 100th cap in front of a Ravenhill crowd will be special. To be getting it on the same night as Chris - who is not only a team-mate but a friend -makes it even more special."
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