Hernanes is talented, no one is denying that.
But there are a number of things to consider:
1. Age: 29, meaning that he has practically zero resale value. Economically speaking, it'd be a terrible deal, not to mention a slap in the face of the so called youth project Thohir has been constantly mentioning since his arrival (especially with the departure of Mbaye)
2. Position: CM. He really isn't a number 10 or a natural AM, having been pushed up front by Reja in 2011. Hell, he's had 1 assist this season in the EL (with 0 in Serie A so far), 7 the season before (including the EL, only having 4 in Serie A), and 4 even before that (with 2 in Serie A, and another two in EL). His passing isn't great, but he does have a casual eye for a final ball or a shot on goal.
3. Inconsistency. Candreva, Klose, and Lulic are probably Lazio's best players (you could maybe even include Radu there), and I'd say that Biglia (despite his poor start to the season), Biava, Dias, Cavanda, and Radu have all been much better than Hernanes this season. Out of 81 shots on goal over the entire season so far, only 32 have made it on target, and from those 32, he's only netted 5 goals (including performances with the Brazil NT as well as with Lazio), which means 6% of his shots have found the back of the net, and 16% of his shots on target have found the back of the net.
He has the technical capability to be performing week in and week out, but he can't manage to put in a string of consistently strong performances, and has undoubtedly been one of Lazio's worst and most inconsistent players this season. I would honestly not be surprised if he initially makes a positive impact at Inter, but even in financial terms it'd be a mistake.
And Cerci would be a MUCH better option for the SS role. Technically capable, entering the prime of his career, and a natural SS/AM/RW.