The 22-year-old scored a hat-trick against Kilmarnock last week, to take his tally to seventeen for the season, and is hoping to receive a Scotland call-up when new boss Walter Smith names his first squad.
Mowbray said: "I am sure Derek has no ambition at this moment to leave this football club.
"There is no need for us to sell him and it would be rather foolish of us to offload him."
Smith is about to name his first squad since succeeding Berti Vogts, having earmarked a get-together spell in Manchester next month to prepare for the tricky World Cup qualifier in Italy at Easter.