Finn Russell and Jonny Gray remain with the Scotland squad for Saturday's Six Nations meeting with Italy but both are fitness doubts.
Fly-half Russell failed a concussion test during Sunday's 27-24 loss to Ireland.
Gray was withdrawn at Murrayfield after suffering a shoulder injury, while fellow lock Scott Cummings has been ruled out with a suspected broken hand.
Zander Fagerson returns from a four-week ban.
The Glasgow Warriors prop was sent off in a one-point defeat to Wales for charging into the head of Alun Wyn Jones while clearing out a ruck.
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Addressing Russell's head injury assessment immediately after the Ireland loss, Townsend said: "It is quite early to tell but Finn was removed from play with a concussion, so it's unlikely he will feature in six days' time."
With Adam Hasting currently serving a suspension, Jaco van der Walt, capped just once, is the only other fly-half in a 35-man training pool.
Grant Gilchrist came off the bench at Murrayfield on Sunday and may well form a new second row with Sam Skinner if Gray is unfit.
Forwards: Ewan Ashman, Simon Berghan, Jamie Bhatti, Dave Cherry, Alex Craig, Matt Fagerson, Zander Fagerson, Grant Gilchrist, Jonny Gray, Nick Haining, Rob Harley, Oli Kebble, Ally Miller, WP Nel, Cornell du Preez, Jamie Ritchie, Sam Skinner, Rory Sutherland, George Turner, Hamish Watson.
Backs: Jamie Dobbie, Darcy Graham, Chris Harris, Stuart Hogg (capt), Damien Hoyland, Sam Johnson, Huw Jones, James Lang, Sean Maitland, Duhan van der Merwe, Ali Price, Finn Russell, Scott Steele, Duncan Taylor, Jaco van der Walt.