What I see with Clyne is someone who will benefit in a team with a deep defensive line with numbers at the back to support him. Defensively, he's very rash (can't believe some are saying he's better defensively than Rafael), and he's heavily reliant on his aggression, strength, and speed to defend. For Southampton, much of his defending is 1-on-1's against wide players. Frequently, he would either charge right at them when they're about to receive the ball or face them, wait for them to try and get past him, and use his speed and strength to hold back the opponent and maintain the ball.
That's not to say he's a defensive mug. He's good at timing his tackles, and as a man-marker, his attributes make him good at stopping most wingers. However, that's all he has that's good enough for this club, IMO. In this case, we already have Valencia who's actually a good man-marker now (compared to before) and is quite reliable, just like Clyne is, against the average/lower-level opposition.
However, I don't see Clyne as being defensively reliable. His awareness and positional sense is quite poor, and his aggressive, man-marking style will easily see us become quite open at the right side against intelligent wide players.
Going forward, he's not bad. Yeah, he may be heavily reliant on his speed, strength, and quickness in beating players, but he's quite good at dribbling, and his deliveries are decent as well. Whenever I've watched Southampton play, he's been one of Southampton's biggest attacking threats after Tadic.
Given that, under van Gaal, we will rarely, if ever, play deep and with many numbers at the back, I won't ever see Clyne as a starter for us. Like Valencia, he can be a good backup player, and I wouldn't mind getting him as that. However, if we are to get a replacement for Rafael as the main right back, we need to aim for better than Clyne. A couple have already been mentioned here, like Fabinho, and I'll also add in Hugo Mallo. In my opinion, Fabinho and Mallo are two of the best up and coming right backs who show great defensive discipline at the moment. Darmian's up there as well, for me.
I wouldn't mind if we get Clyne as a squad player who van Gaal can shape into a great defensive player, but that's only if van Gaal doesn't sell Rafael and entrusts him the right back position in the first 11. Otherwise, we'll need to aim for better than Clyne.