And when Sale lock McKenzie suffered a dislocated shoulder on Thursday, Ryder was handed the chance to challenge for a place in the Calcutta Cup opener against England on February 4.
He said: "I'm delighted and proud to have received the call-up to the squad.
"First and foremost, my focus is on helping Glasgow Warriors win a massive Heineken Cup match in Bath, but I'm really looking forward to joining up with the rest of the Scotland squad, putting in a lot of hard work and challenging for a place."
Scotland head coach Andy Robinson admitted he was disappointed McKenzie would not be available but is adamant Ryder is well worth his call-up.
Robinson said: "It's a blow for Fraser, but I've always stressed the importance of performing on Scotland A duty and consistently for your club.
"Tom has done this and deserves this chance."
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