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Forgive the fuzzy image, but the video was not great quality, but I think this frame is the best to see both the two United player's intents and the two key Saints player's positions. Firstly, the intents, Januzaj is clearly intending to go inside whilst Evra is looking to run towards the by-line as he eventually did. Meanwhile Clyne is positioned to protect a run down the line, he is a yard further towards the by-line than Januzaj with his strong foot firmly pointing in that direction and he has his back to the direction Januzaj wants to go. From this position he can shut out anything wide but has a bad starting position and would lose a yard if Januzaj were to knock it inside. Wanyama is occupying exactly the area Januzaj wants to get to, and with him in that position (And Adam also closing) the only thing Januzaj could do if he came inside without creating a Ribery-like moment of skill would be pass backwards or cut back outside. The only other option he would have would be if Clyne were to leave his position he could try and cross the ball, which our two CBs have perfect positions to clear, or lastly to follow Evra down the line which would just create a traffic jam. So all options are covered except from Evra's fateful overlap, which Clyne has both the best position and skillset to cover. He clearly from there already has Evra in his sight-line and has the perfect position to immediately close out any pass to him, with him having a three yard headstart and great closing speed thus being the ideal man to track that run. Phew, that would have been much easier with one of Gary Neville's fancy mega touchpads.
So there's my explanation, if that's being a Wanyama apologist I... apologise.