"We shouldn't kid people we would," Ben Wallace told the Spectator magazine. "The Ukrainians are aware of that."
Russia has moved thousands of troops near Ukraine's eastern borders, leading Western powers to urge President Vladimir Putin to de-escalate tensions.
Moscow has denied plans to attack and has asked for urgent talks with the US.
On Friday, Russia demanded strict limits on the activities of the US-led Nato military alliance in countries in Eastern Europe.
The alliance was originally set up to defend Europe against possible threats from the former Soviet Union.
In an interview with the Spectator, Mr Wallace said Ukraine was "not a member of Nato, so it is highly unlikely that anyone is going to send troops into Ukraine to challenge Russia".
"That is why we are doing the best diplomatically to say to Putin don't do this," he said, adding that "severe economic sanctions" were the most likely form of deterrent.
Mr Wallace has previously said that the UK stands "shoulder to shoulder with the people of Ukraine" and remains determined to support them.
Earlier this week, UK Prime Minister Boris Johnson warned Mr Putin of "significant consequences" of any "destabilising action" in the region.
Ukraine shares borders with both the EU and Russia, but as a former Soviet republic it has deep social and cultural ties with Russia.
The Kremlin has accused Ukraine of provocation with its ambition to join Nato.