Antony Beevor's Stalingrad contains some supporting material on perceived weaknesses of the Red Army esp. following Stalin's domestic purges:
Such confidence [in the success of Operation Barbarossa] was, in many ways, understandable. Every foreign
intelligence service expected the Red Army to collapse. The Wehrmacht
had assembled the largest invasion force ever seen [...]
Hitler's conviction that the Soviet Union was a 'rotten structure'
that would come 'crashing down' was shared by many foreign observers
and intelligence services. Stalin's purge of the Red Army, which had
begun in 1937, was fueled by an inimitable mixture of paranoia,
sadistic megalomania and a vindictiveness for old slights dating back
to the Russian civil war and the Russo-Polish war.
Altogether, 36,671 officers were executed, imprisoned or dismissed,
and out of the 706 officers of the rank of brigade commander and
above, only 303 remained untouched [...]
The most prominent victim was Marshal Mikhail Tukhachevsky, the
leading advocate of mobile warfare. His arrest and execution also
represented the deliberate destruction of the Red Army's operational
thinking, which had encroached dangerously upon Stalin's preserve of
Two and a half years after the purge began, the Red Army presented a
disastrous spectacle in the Winter War against Finland. Marshal
Voroshilov, Stalin's old crony from the 1st Cavalry Army, displayed an
astonishing lack of imagination. The Finns outmaneuvered their
opponents time after time. Their machine gunners scythed down the
massed Soviet infantry struggling forward through the snowfields. Only
after deploying five times as many men as their opponents, and huge
concentrations of artillery, did the Red Army begin to prevail. Hitler
had observed this lamentable performance with excitement.
Japanese military intelligence took rather a different view. It was
about the only foreign service which did not underestimate the Red
Army at this time. A series of border skirmishes on the Manchurian
frontier, which culminated in the battle at Khalkin-Gol in August
1939, had shown what an aggressive young commander, in this case the
forty-three -old General Georgy Zhukov, could achieve.
As for the key sentence ("Hitler had observed ...") Beevor does not point to further sources, so you would have to believe his word as an expert that he certainly is.