Eovaldi did throw only 13 pitches last night, and with an off day today, would still be available for a Game 4 start.
I'm guessing (which is all I can do) that Cora likes Eovaldi in that late inning relief role more than either Barnes, who's had some control issues at times during the season, and Brasier, who for all we know could simply be riding a 40 inning hot streak.
There is a time to worry about Game 4, and that is after Game 3. There are options if Eovaldi is unable to start in Game 4: EdRod or Pom as "openers", Sale and/or Price pitching an inning or 2, using Kelly for 6 or 9 outs, etc. And there are too many things that can happen in Game 3 that can change things anyway (extra innings, blowout win or loss).