I can see both sides of the argument. Farrell has to manage the marathon, not the sprint. He can't be in a 'win today's game at any cost' mode this early in the season.
That said, I am a believer in the philosophy that you should not always save your closer for the 9th inning. Sometimes the true 'save' situation comes in the 7th or 8th inning. Last night's game was a perfect example. There is no sense in saving your closer for a close situation in the 9th that may never come. If the Sox had blown that game in the 8th with Kimbrel sitting on the bench waiting for the 9th inning save which never came, what a waste that would have been.
Kimbrel saved the game in the 8th, which was the true 'save' situation of that game. It turned out that we didn't need him in the 9th. But if we had, there are times when Farrell could use Kimbrel for more than one inning, and when he can't, he'll have to take his chances with another reliever to close out the game. He can use Kimbrel in the 8th without overusing him.
But again, it doesn't make a lot of sense to save your best reliever for a 9th inning situation that may never materialize. Use him when the game actually needs to be 'saved', be it the 7th, 8th, or 9th inning.