Gabriel Sara was superb for us - not the quickest and can sometimes look laboured, but this is due to his style rather than lack of work-rate (he's certainly not lazy and can cover ground).
He's got great feet, as you'd expect from a Brazilian, and a superb pass on him. This goal
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(1m:27s) demonstrates his quick feet and calmness when under pressure, and then this goal
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(1m:55s) demonstrates his passing range and assist potential.
He's be a perfect fit for your side, and plays best as a play-making CM. He can tackle and defend but it's not really his game, he'd work well in tandem with Xhaka. Not sure if you need a corner-taker, but he's decent at those as well.
Finally, he's a top human-being and certainly not the type to cause any issues within the squad. He deserves a shot in the EPL and I believe a club like Sunderland would be the perfect move for him, given your current league standing. Anything under £20m would be an absolute bargain, £20-30m would be decent, £30-35m would be his true value, £35m+ and you've probably over-paid.
Hope that helps.