Why A’s are using a reliever to pitch the first inning

When the lineup was posted for Saturday’s game, pitcher Daniel Mengden wasn’t listed as the starter even though he had been scheduled to start.
The starter: reliever Liam Hendriks.
The A’s are the latest team to pull a pitcher out of the bullpen to open on the mound and pitch an inning or two. It’s the first day of September, so it’s the first day roster limits are 40 instead of 25. Oakland’s bullpen includes 14 relievers.
That includes Mengden, at least for a day. The plan would be for him to follow Hendriks.
“With the starters going down, we’re going to have to use our bullpen anyway,” said manager Bob Melvin, whose latest rotation casualties were Sean Manaea and Brett Anderson.
Source: Sports