Quins say Robshaw failed a late fitness test. He has not played since suffering an ankle injury during the league victory over Worcester three weeks ago. England skipper Robshaw was arguably the highest-profile omission from a 37-man British and Irish Lions squad announced 10 days ago to tour Hong Kong and Australia this summer.
"I can't think any person could have done more," Quins rugby director Conor O'Shea said, reflecting on Robshaw's Lions heartache. "I think he has been very hard done-by."
He added: "Maybe sometimes you pay for being under the microscope, whereas some players can fly under the radar in non-event games.
"I told him selection is opinion. I think it's very easy to focus on what people can't do or what people do better, rather than focus on what they can do. All I see is a player who has captained England in adversity and to a record win against the All Blacks. He was magnificent in the autumn internationals. He won two out of four man-of-the match awards."
Despite finishing third in the league and missing out on play-off home advantage, title holders Quins will arrive at Welford Road following three successive wins against Leicester, including last season's Premiership final when they triumphed 30-23 and Robshaw scored a try.
Leicester, though, boast an imperious play-off record and they are bidding to book a ninth successive Premiership final appearance. They have won their last 12 semi-finals in all competitions.
"We've played a lot of rugby in a long season, but now it is just about getting the detail right and bringing your best game to the occasion," Tigers rugby director Richard Cockerill said. "We've been in good form and had some good momentum in the run-in, and we need to maintain that high level against a very good Quins side."
Saracens host Northampton in Sunday's play-off action, with Saracens unbeaten on six previous appearances as their new Allianz Park home. Saints, meanwhile, have featured in five previous Premiership semi-finals, but lost the lot.
"Under Jim Mallinder and Dorian West, Northampton have been a very consistent side in recent seasons," Saracens rugby director Mark McCall said. "Like us, they have regularly made the semi-finals of this competition and we have met many times along the way, both in league and knockout rugby. Both teams know each other well, but both teams also have players who can turn a match in an instant from any part of the pitch and in any position."
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