lesser, lessor

Lesser or lesser means the smaller or least important of two alternatives--
He thought his choice was the lesser of two evils.
The windows are a lesser concern. Let's get the roof fixed first.

Lessor or lessor ( rhymes with lesser, almost ) means the person who rents or leases something to someone else--
Put the landlord's name in the space for the  "lessor".
The lessor leases the property to the lessee.

Now that you know that, you can say--
"Calling the lessor was a lesser concern; the plumber was more important."