There are a few things in here that are fun to think about.
In the situation you describe, the answer is often "yes" and that's how you end up with evolution. Let's say you have a gene that makes a protein that digests glucose. And then one day, your cell messed up when replicating and accidentally made an extra copy of that gene. Well now you have an extra copy of that gene that isn't under purifying selection. It's redundant. It can mutate but as long as you have the first copy, you're ok. And eventually it mutates away from being good at digesting glucose. It can do it a little bit, but it's not great. Maybe it's 20% as effective as it originally was. But you have another gene that's still 100% effective so you don't even notice.
Now we have a protein that really doesn't do anything bad... It just doesn't do much good either. And since it isn't subject to purifying selection, every round of replication it keeps mutating. Until all of a sudden, it mutates into something that can digest lactose. Now, you have an evolutionary advantage from a protein that first had to get bad at binding glucose, before it could benefit you. But evolution has no foresight, so it didn't know. So it took getting rid of purifying selection to make it happen. But now as you come to rely on lactose, that protein will wind up back under purifying selection and become "fixed".
So now let's consider an alternative situation. You only have one gene that can digest glucose. If it mutates to be 20% effective, you will at best grow only 20% as fast as your competitors. Maybe you even die and become an evolutionary dead end. In that case, it is an extreme disadvantage to have a protein that can't do its job reliably, and organisms that don't have a malfunctioning variant will grow better and pass on their genetic material to more offspring, until you are eventually outcompeted and go extinct.
We can also imagine another scenario. Your glucose digesting protein mutates into something that can still bind glucose, but cant digest it. Then the glucose remains stuck to the protein, producing no energy, and becomes a waste for the cell. That is actively harmful and will likely kill the cell very quickly.
In the situation you describe, the answer is often "yes" and that's how you end up with evolution. Let's say you have a gene that makes a protein that digests glucose. And then one day, your cell messed up when replicating and accidentally made an extra copy of that gene. Well now you have an extra copy of that gene that isn't under purifying selection. It's redundant. It can mutate but as long as you have the first copy, you're ok. And eventually it mutates away from being good at digesting glucose. It can do it a little bit, but it's not great. Maybe it's 20% as effective as it originally was. But you have another gene that's still 100% effective so you don't even notice.
Now we have a protein that really doesn't do anything bad... It just doesn't do much good either. And since it isn't subject to purifying selection, every round of replication it keeps mutating. Until all of a sudden, it mutates into something that can digest lactose. Now, you have an evolutionary advantage from a protein that first had to get bad at binding glucose, before it could benefit you. But evolution has no foresight, so it didn't know. So it took getting rid of purifying selection to make it happen. But now as you come to rely on lactose, that protein will wind up back under purifying selection and become "fixed".
So now let's consider an alternative situation. You only have one gene that can digest glucose. If it mutates to be 20% effective, you will at best grow only 20% as fast as your competitors. Maybe you even die and become an evolutionary dead end. In that case, it is an extreme disadvantage to have a protein that can't do its job reliably, and organisms that don't have a malfunctioning variant will grow better and pass on their genetic material to more offspring, until you are eventually outcompeted and go extinct.
We can also imagine another scenario. Your glucose digesting protein mutates into something that can still bind glucose, but cant digest it. Then the glucose remains stuck to the protein, producing no energy, and becomes a waste for the cell. That is actively harmful and will likely kill the cell very quickly.
So to answer your question: it depends