user: Q: Pienaar, who is leaving Ulster at the end of this season, sustained the injury in the warm-up before Saturday's defeat by the Ospreys.
The South African, 33, started Saturday's contest but was forced off after 20 minutes.
Ulster's hopes of reaching the play-offs semi-finals now look remote.
Les Kiss' side are now five points behind fourth-placed Ospreys and only a huge bonus-point victory over the leaders this weekend - allied to a heavy Ospreys defeat - would see Ulster sneaking into the semi-finals.
Pienaar will round off his Ulster playing career in the game the Barbarians at Kingspan Stadium on 1 June.
Stuart Olding will miss the Leinster game after picking up a leg injury in Saturday's defeat while Charles Piutau also picked up a knock in the weekend game.
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A: Ruan Pienaar may have played his final Pro12 game for Ulster as a back spasm means he is a doubt for Saturday's contest with Leinster.


Q: Translink is "inviting the public to tell them how they use local NI Railways services to help plan network changes to be introduced this summer".
It focuses on early morning, evening and Saturday services on the Bangor-Portadown, Newry and Larne lines.
Unite said it was a "rubber-stamping exercise designed to provide political cover for damaging austerity cuts".
The union's Davy McMurray said there were "no clear proposals" in Translink's questionnaire for passengers.
"The public information meetings have not been publicly advertised and are timed to exclude the public; for example that (the meeting) in Portadown runs from 7.30am-10am," he said.
"What's worse is that many towns which will be affected by cuts such as Lisburn, Newry and Whitehead have no planned information events at all."
Mr McMurray said it was a "bogus consultation exercise geared to providing a fig leaf for their preordained but undisclosed cuts to bus and rail services".
He said Unite representatives at Translink would meet early next week "to agree an industrial action strategy with the aim of protecting our members' jobs and defending the integrity of Northern Ireland's public transport system".
Translink marketing executive Ciaran Rogan said the consultation was "part of our overall strategy to address current funding challenges".
"An Ulsterbus services consultation is already under way and last week we also launched a voluntary exit scheme that will cut our management, overhead and administration costs by £3.1m annually," he said.
"The information we get from passengers will be very important to us; we want to continue to offer accessible and good value services where customers need them while ensuring we remain efficient and sustainable for the future."
He added: "It is extremely important that our customers participate in this process and contribute to shaping the future of our local train services."
Translink is holding a series of public meetings as part of its consultation that runs until 1 May:

A: A union is considering industrial action over a Translink consultation on railway changes.


Q: But as Fifty Shades of Grey had its UK premiere campaigners protested against the franchise claiming it glamorises domestic abuse.
However, the cast, director and author have all told Newsbeat that's not true.
Some threw flyers while one protester was removed after attaching a banner reading "Mr Grey is a Rapist" to the railings.
Natalie Collins, who leads the 50 Shades is Domestic Abuse campaign, told us a separate group of protestors were responsible and removed.
The 30-year-old also explained what her campaign was about: "We set up the campaign two and a half years ago when the books came out.
"We felt really concerned about the way the books romanticise abuse and glamorise an abusive person.
"We are really concerned about the way that he stalks her, he coerces her into sex, he coerces her into giving up her car, he harasses her, he's emotionally abusive.
"We are against that, we're not against the kinky sex, we are not against BDSM at all."
The trilogy of books the film is based on follow the relationship between student Anastasia Steele, played by Dakota Johnson, and emotionally-damaged billionaire Christian Grey, played by Jamie Dornan, who enjoys controlling sexual encounters.
Campaigners say some of the sexual behaviour carried out by Dornan's character crosses into abuse.
When asked about the criticism, Dakota Johnson told Newsbeat: "It's exciting to be involved in something controversial that some people like and some people don't.
"I wonder if they wouldn't mind waiting until they see the movie. I don't think it's that way at all."
Director Sam Taylor-Johnson said: "I don't think you can judge something without seeing it.
"What Dakota and I did throughout the journey of Anastasia was to empower her.
"Every sexual encounter that she has with Christian is one that she's gone into willingly and complicity until the moment he crosses a line.
"And when he crosses that line it's a very firm no and she has the final word - she has all the power and he's the vulnerable one so I don't think there's any glamorisation of it."
Four and a half million tickets have already been sold in the 39 countries where Fifty Shades of Grey will be released this weekend.
While children as young as 12 can watch Fifty Shades of Grey in France - it has an 18 rating in the UK.
Seventeen-year-old Iriana has joined the 50 Shades is Domestic Abuse campaign.
She said: "I think it's a disgrace they are still showing this. My friends think it's OK for girls to be part of this and it's a new experience in their opinion.
"It doesn't matter if it's an 18 - it's the fact it's degrading women."
Security removed a banner and flyers from the grey carpet moments after the book's author EL James had stopped to pose for photos with fans.
When we asked her about the criticism she said: "Why do you think there are so many women here if it's about domestic abuse and domestic violence?
"Don't get me started. No it's not - OK?"
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A: It's already taken Â£1m in ticket sales and it's only just been released in cinemas.


Q: The shops include music and book store Rare & Racy which has traded on Devonshire Street since 1969.
Proposals approved by Sheffield Council will see apartments, a restaurant and a shop built on the site.
Hundreds of placard-waving protesters gathered outside Sheffield Town Hall on Tuesday ahead of the decision.
Seven councillors voted in favour of the plans, with three against and one abstention.
Adam Murray, director of Coda Planning Ltd which submitted the application, said: "The simple truth is that these buildings are in a state of disrepair.
"This means that if no action had been taken then in the longer term the buildings would not be able to be occupied and would have become a serious blight to the area."
The shops will be replaced by a three-storey building which will include a shop unit, restaurant, two studio apartments, six single bedroom apartments and four two-bed apartments.

A:

assistant: Plans to demolish a row of shops in Sheffield have been approved despite a 20,000-signature petition against the scheme.