Did you have the HSA in 2017? If so, then you are allowed to ask your HSA custodian at any time for a reimbursement of any qualified medical expenses paid after that HSA was created. It does not matter if you reported the HSA on your 2017 return or no - the HSA custodian has filed reports with the IRS showing that the HSA existed.
In fact, it is OK to ask for the reimbursement even years later after the expense, so long as the HSA existed when the original expense was incurred.
Yes, do amend your 2017 return to cause the form 8889 to be added to your 2017 return. It may make no difference to your tax due, but it will square up your return with the copies of the forms that the IRS has received from your employer and your HSA custodian, which avoids letters of curiosity from the IRS.
