Hi Jorge! That's actually a pretty tough call. Tatis is indeed getting hot, and it's far from certain that Nevin will return at full strength. I'd decide this by your team's current status: if you already have a viable 3b, go with Nevin, who has a higher ceiling. Also choose Nevin if you're trailing in the standings and need to take some risks.
If you're already at or near the top of the standings, and/or have no other 3b, stick with Tatis, the safer option.