I definitely don't think that the likes of a 42-year old journeyman or Tanner Rainey can be trusted to be a consistent setup guy down the stretch in a pennant race.
If Rizzo is looking for a current closer who could become an 8th inning guy here, here are the best options:
(age) current salary/contract
Shane Greene (28) - 1 yr/$4m, arb eligible, FA 2021
Ken Giles (30) - 1 yr/$6.3m, arb eligible, FA 2021
Will Smith (30) - 1 yr/$4.23m, FA 2020
Alex Colome (30) - 1 year/$7.33m, arb eligible, FA 2021
Each would probably cost some top tier prospects which I'd be hesitant to trade for guys who are arb eligible this winter with only 1 year of control left or Smith who would be a rental. Rizzo could be thinking the middle relief route which would cost less in terms of $$$ and in addition could get one or two for lesser prospects.
The problem is, of course, that with the number of teams in the playoff hunt around baseball there is likely to be a bidding war for those guys which would drive the price up. I'm sure Rizzo is hedging his bets that there will be some solid setup guys available as well who would come cheaper.