If workers don't put in the effort, you fire them, or demote them, or pay them less, or give them less hours, or... all of the same things employers do today.
Productivity from fear of loss, I would expect, is inferior to productivity out of hope of gain. If I am motivated from fear, I do not want to be an outlier (I want to be nestled in the group), and I only want to make defensible decisions ("It's not my fault, see?"). And then there's the health effects from stress, the breakdown of group dynamics from members turning on each other, the issues and costs of enforcement of fear-generating policies... So let's not underestimate the costs of fear on productivity.
There are also employers who do not fire employees out of generosity - "I don't want to fire that guy, because he really needs this job". Now you can - or suspend without pay or whatever - because you're not throwing them on the street.
I can definitely deny that what you propose is a real (expected, since this is all theoretical) cost, and you should, too.
In the current system bosses are sometimes hesitant to fire people if they know they have a family to support. Knowing people have UBI to lean on there's much less guilt about letting people go.
It'll also kill of a load of businesses where there's a bulk of minimum wage staff that could work for themselves.
UBI will guarantee you won't die by something stupid and terribly like starvation, but if you want a nice condo, nice car, better entertainment, or maybe travel -- well guess what you have to suck it up and do? And poor workers won't be tolerated. I can only assume it would become easier to fire people if doing so won't immediately place them in danger as that danger would have already been mitigated.
As a rule, I think it's best to be aware that it can almost always be possible to be worse some for people than you think it can.
The company almost by the nature of the relationship is telling a person what to do. It is about the only necessarily hierarchical relationship left.
But beyond that, in your average company, the company can afford to do without a certain employee for some time (their income might drop from 100 to 99), but an employee can't do with a company (their income would drop from 100 to 0). So even beyond the nature of the relationship (recognised by law), there's other informal power relationships going on as well.
So you might worry about bad workers, but you can hardly complain about abusive workers.
Of course, we could argue that workers should be more motivated by the promise of a good reward, but it looks like evidence from behavioral economics suggests that fear of loss is approximately twice as powerful as the hope of gain. So let's not underestimate the power of fear to generate productivity.
By the way, I am generally supportive of the UBI, just wanted to point out a potential cost. Over the long run I think the UBI's benefits would exceed its costs, but I can't deny that there are real costs to an UBI (beyond the simple direct costs).