Fernando is a specialist player, his utility resides in staying in front of the CBs and allowing more freedom to other midfielders. He's sublime in this defensive role, with excellent reading of the game, positioning, and good tackles. He doesn't offer much with the ball on his feet. You'll have to change your tactics to accommodate him or I think you'll suffer the same disappointment you had with Javi García. Fernando always excelled at the back of a midfield triangle here, and fit like a glove in our formation, but has no experience in other roles. This season when we inverted the triangle he looked a bit lost.
Mangala seems to have the tools to become a beast of a defender, but still lacks consistency. This season was poor from everyone, with a lousy manager at the helm so I think it was a missed step on his development. He's not a 40m defender yet, I think there's a good chance he will soon, but his valuation here is more based on future potential than actual ability.