It's not unplayable i think , but most likely an early 6.Bg5! pin is a little nasty for black. Usually black plays Nf6 only after white's g3 so that ...h6 can't be meet with .Bh5.
Tarrasch Defense
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I believe a possible answer is g3 , which is what Akiba Rubinstein introduced in the early 20th century against the Tarrasch if i'm not mistaken
The idea is to fight against d5 pawn (which becomes isolated usually) while making sure that you win the fight for the c file and prevent Black to setup active counterplay on the Queenside
Ashigaru, g3 would be the standard plan, the move white usually plays against 5...Nc6, and is of course perfectly good also after 5...Nf6. But if he wants to punish the unusual move order, he should play 6.Bg5 which immediately threatens the distruction of black's centre.
My opponent threw me off by playing a line I wasn't familiar with. What's the best response to 5...Nf6? Is this line published anywhere or is it totally off book?