You say RB/LB is not a position we needed someone in. I disagree. My only problem with Serge is his mental issues. The dude does absolutely stupid dumb things. He is the Balotelli of full backs. He comes with everything both good and bad.
Personally I thnk there are higher priorities. If we fill all the other priorities and then have cash left over then sure, why not. However I'd let Trippier and KWP have this season to prove themselves with the knowledge that we hopefully have Dier and Sanchez as back-ups if things gone wrong. Then buy a new RB next summer if Trippier doesnt step up and KWP doenst look like he'll make it.
for what its worth, I think our priorities should be a CB (sanchez), Barkley and some other attacker with some pace or skill and who could ideally fill in up front