Scotland wing Nikki Walker is expected to be sidelined for a month after suffering a stress fracture of his right fibia.

He joins Wales scrum-half Mike Phillips, who is nursing a serious knee injury, in missing the Ospreys’ trip to Vicarage Road.

Phillips will not play again until next season, while Walker will also miss the Cup final against Leicester at Twickenham next Saturday.

Justin Marshall takes over from Phillips, with Johnny Vaughton replacing Walker. Ospreys head coach Lyn Jones though, still fields 12 members of Wales’ Grand Slam squad.

The Ospreys smashed Saracens 30-3 in a Cup semi-final just two weeks ago, but Jones insists his players will not be reading too much into that result.

Jones said: “Sunday’s game is sure to be a cracker, with two good sides fully aware of what is at stake.

“It is always a strange situation when you face a side twice in such short succession, but it’s fair to say that we won’t be taking anything for granted, despite the manner of our victory at the Millennium Stadium two weeks ago.

“Back-to-back matches can be completely different affairs, as our own experiences against Ulster in this tournament in December show.

“We recorded a comfortable win at the Liberty Stadium, but it was a much tighter affair at Ravenhill the following week.

“The message that we’ve been getting across to our players all week is that they must stay focused, keep their confidence in check to avoid any complacency and forget what has happened in the past.”

Saracens boss Alan Gaffney has gone for experience, recalling centre Andy Farrell, prop Cobus Visagie and flanker Richard Hill, while wing Richard Haughton also features as a fourth change from the side that crushed Guinness Premiership relegation favourites Leeds Carnegie 66-7 last weekend.

Gaffney said: “We got the kind of performance we wanted against Leeds, and we have to take that into Sunday’s game.

“We know the Ospreys will be a completely different opponent, but the fact is we have to go out on the front foot and attack them.”

Cardiff Blues include former Toulouse back Gareth Thomas in midfield for their tough semi-final clash against the three-time European champions at Le Stadium on Sunday.

Thomas forms a centre partnership with Tom Shanklin, while Dai Flanagan has been handed the key fly-half role as the Blues target a surprise victory.

Toulouse have named France scrum-half Jean-Baptiste Elissalde in the number 10 shirt - he will be partnered at half-back by Byron Kelleher - with veteran lock Fabien Pelous captaining the team.