You can, but go carefully and get the timing right. A performance concern doesn't vanish because someone's gone off sick, and you're allowed to deal with it. What you shouldn't do is push on with a formal process as if nothing's changed. Stress can count as a disability, and ploughing ahead can look like you caused the absence or made it worse.
The safer route is to get occupational health or medical input on whether they're well enough to take part, and what support they'd need, before you carry on. Keep in touch like a human, agree how and when you'll pick it back up, and write down the care you took. Handle it with patience and it's manageable. Rush it, and it's one of the quickest ways to turn a performance issue into a discrimination claim.
Every situation has its own details, and those details change the right answer. For a read on yours, take the free Employee Situation Check or book a call.
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How to handle this, in a couple of minutes
Samantha talks through the practical steps.
General guidance to help you decide your next step, not legal advice. Employment law changes, so always check the current position for your own situation.