JAMES Davies has been prescribed suppositories to cope with a mystery injury and now the flanker intends to be a pain in England's backside to boost his World Cup chances.

The flanker will win his fourth cap when he lines up for Wales at Principality Stadium this afternoon (kick-off 2.15pm), one of three changes to the XV that lost 33-19 at Twickenham last Sunday.

The openside joins forces with Dragons duo Aaron Wainwright and Ross Moriarty in the back row and knows that he cannot waste a chance to impress.

With Warren Gatland set to take five loose forwards to Japan and Josh Navidi and Justin Tipuric also in the mix, it appears from the outside that Davies is up against his Scarlets teammate Aaron Shingler, a blindside with the ability to cover lock, for the last spot.

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The 28-year-old, who will line up alongside older brother Jonathan for the first time in Test rugby, is desperate to grasp a chance he feared wouldn't come.

Davies has been struggling with a back problem that the medical team have struggled to get to the bottom of.

It has impacted his efforts in training and the opensides has been taking Diclofenac suppositories, but he is determined to not let it derail his late charge for the World Cup.

"It's ongoing really, but nerve issues down the legs and stuff. It's both the top and lower back," he said.

"It's one of those things, something I'm dealing with. It won't hold me back on the weekend.

"Throughout my career I have had an ability to deal with niggles and stuff. When you play openside you're going to get knocks all the time.

"This is one of the hardest injuries I've had to get over but I'm in a good place mentally and am sure it'll be alright."

The fierce competition in the back row means that Davies has had to get used to being patient but his big breakthrough came last summer when he impressed Gatland with his exploits against Argentina.

Just when the flanker, a silver medallist with Team GB at the Rio Olympics, thought he had worked his way in he suffered injury misfortune.

Knee ligament damage on club duty meant he missed the autumn Tests and then a foot problem cost him a place in the Six Nations.

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"You just want to kick on from being in a good place. It hasn't happened, but that's rugby, that's sport, it's about how you come over those hurdles and where you end up," he said.

"For me to be sat here, if you told me I'd be starting a Test for Wales a month or two ago I would have disagreed with you, but now I'm here and am looking forward to it."

Even in the absence of Taulupe Faletau (shoulder), Ellis Jenkins (knee) and Thomas Young (foot), Davies is up against it to earn a spot after falling behind the Grand Slam quartet of Navidi, Tipuric, Moriarty and Wainwright.

"I said to Gats this week that the back row group is so impressive that if you didn't get called up you would have no arguments with who they went with," he admitted.

"The opportunity has probably come a bit sooner than I expected, having not played all year but it is about me stepping up and making sure I do my job right, and taking this opportunity."

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And after a career of being the underdog, Davies is relishing the chance to give Gatland, who admitted he had doubts about whether the flanker could step up to Test rugby, a headache by impressing against an imposing England pack.

"I have been doubted throughout my career because of my size. I remember going to university and not getting an academy spot because I was too small etcetera.

"I thrive on proving those people wrong, it keeps me going every day. I love it, to be fair. People doubting me spurs me on.

"You could say perhaps when you are not getting selected for the squads that your face might not fit at that time, but you just try to keep knocking on the door.

"Once I got into the environment I think I changed people's opinions. I find that once I get the opportunity I take them. I'm just pleased that they have changed my minds and that my face fits now."