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If you think about it, the Eagles have only had a few options on the wing for the last year. James Paterson is a solid option but has struggled with injuries. It's really been Hume and Ngwenya that have been the options. The Eagles need to build depth at the position if they want to avoid a repeat of last summer when they missed Ngwenya for certain matches or needed to rotate players. There are other options, like a Blaine Scully, Chris Wyles, or Folau Niua, but their best positions are probably elsewhere.
Another factor is Ngwenya's current situation is the form of his club, Biarritz. They are having a terrible season with only one win and nine losses. They are nine points behind their closes opponents and 13 points away from clearing the relegation zone. With the season nearly half over Biarritz can't afford to lose one of their better players. Unless they can pick up points now they are going to be in a dogfight at the end of the season. If you are Tolkin would you rather not have Ngwenya now or not have him in the Spring for World Cup qualifying? Showing a little generosity to his club now could pay off down the road.
In talking with Tolkin it's clear that Ngwenya is still very much in the Eagles plans moving forward. Right now it's simply the case of trying to reward younger players for playing well. Ngwenya's non-selection also doesn't come as a result of hime going off on his own as has been reported elsewhere. He definitely has a tendency to play loose at times but often that comes out of a frustration of some of his teammates not being able to play at the pace--both physically and mentally--that he is accustomed to in the Top 14. As has been seen, when the Eagles are on their toes playing at a high level Ngwenya fits right in. So all in all it's not that big of an issue on the team and you can expect to see him in March.