Flanker Ferris suffered his injury in training on Tuesday
Ulster and Ireland flanker Stephen Ferris will miss the rest of the British and Irish Lions tour of South Africa because of a knee injury.
The 23-year-old tore the medial collateral ligament in his right knee in training on Monday
Ferris had impressed on the tour and was in contention for the number six jersey for the first Test against South Africa in Durban on 20 June.
Welshman Ryan Jones has been called up to replace Ferris.
Jones said: "I am anxious to help the Lions in any way I can."
Lions head coach Ian McGeechan said: "Ryan is on the standby reserve list and has been playing over the last few weeks for Wales in the USA and Canada."
Ospreys back-rower Jones was also left out of the 2005 Lions tour of New Zealand before being called up as a replacement, and McGeechan said he had shown his worth then.
"On that tour he showed how competitive he is as an individual by forcing his way into the Test squad for all three matches against the All Blacks," said the coach.
"He will fit right in as he knows a lot of the players."
The 28-year-old, a double Grand Slam winner with Wales, said: "I recognise how this is a great honour for me and my family.
"I am delighted to have received this call-up and I am just really looking forward to joining the squad later this week.
"Obviously it is disappointing for Stephen Ferris who is a superb player and I wish him a quick and full recovery from his injury."
Jones is scheduled to arrive in South Africa on Thursday 11 June after flying out with newly appointed Lions kicking coach Neil Jenkins. They will join the rest of the squad in Cape Town.
Ferris made his first Lions appearance as a scoring substitute in the 74-10 win over the Golden Lions in the second tour match and furthered his chances of a Test jersey when he impressed - and scored again - as a starter in the 26-24 victory over the Free State Cheetahs on Saturday.
But after Tuesday's scan on the injury to his right knee, he was ruled out for four to six weeks.
Ospreys elite performance director Andrew Hore said Jones thoroughly deserved his chance with the Lions, even if it had come about in an unfortunate way.
Obviously it's disappointing for Ferris and the fact that he has to leave the Lions squad before a Test has even been played but it's a fantastic opportunity for Ryan
Ospreys elite performance director Andrew Hore
"I'm sure if he goes over to South Africa now he'll make a fantastic impression now," he said.
"It's also a tribute to the way he's conducted himself since hearing he wasn't in the initial squad.
"He's worked hard, led the Wales team well in America and Canada and he's been rewarded which is fantastic.
"But there's always an element of luck. Obviously it's disappointing for Ferris and the fact that he has to leave the Lions squad before a Test has even been played but it's a fantastic opportunity for Ryan.
"The players work their heart and soul to make the Lions and so they should.
"Outside the World Cup, it's one of the most prestigious things you can get involved in so of course there was disappointment and as I say, the way he's conducted himself around that was outstanding."
Jones was initially overlooked for Ireland flanker Alan Quinlan, but before the tourists left for South Africa the Munster man was banned until September for making contact with the eye area of Leinster captain Leo Cullen in the Heineken Cup semi-final.
The Lions also lost Ireland hooker Jerry Flannery, who chipped a bone in training before the tour, Welsh centre Tom Shanklin, who dislocated his shoulder playing for Cardiff Blues, and Ireland scrum-half Tomas O'Leary, who broke his ankle playing for Munster.
England's Tom Croft replaced Quinlan, Scotland's Ross Ford came in for Flannery and Scotland captain Mike Blair took over from O'Leary.
Shanklin was not replaced but Wales fly-half James Hook was draughted in as cover for team-mate Leigh Halfpenny, who has been struggling with injury but remains on tour.