Romania would by no means be a push-over and the Soviets are going to be suffering badly:
- execrable Soviet organization (this cannot be stressed enough)
- terrain that is hilly, somewhat forested and has multiple rivers running along the front, with little usable infrastructure
- a Romanian army that will have pretty good morale, a considerable artillery park and decent numbers, fighting on home ground
As a point of reference, Romania managed to defeat the Red Army formations right in front of them and take back Bessarabia mostly (though not completely) without German asisstance in '41.
Working against Romania is the fact that they will have to keep forces deployed against Hungary and Bulgaria as well. (And if they attack as well, Bucharest will sue for peace,regardless of the momentary military situation)
The diplomatic situation must also be considered - Germany was deadly afraid of the Soviets taking the oilfields, and would certainly pressure Stalin not to advance too far; Italy meanwhile was being actively courted by Romania, who was hoping to gain a new patron - OTL, Mussolini sided with Hungary (and Germany) against Romania when it came to piciking, but here things are different, and the outcome might not be the same.
Lastly, there is a decent chance that Romania resists long enough for Hitler to give up on the thought of invading Britain and deciding on Barbarossa instead, which would further complicate the diplomatic situation.
What is certain in my opinion is that the Germans would under no circumstances invade themselves, because while the Soviets could stomach Romania preemptively blowing up the oilfields and refineries, Germany could not