Timely Reinforcements puts a real strain on aggro or Burn. Boarding in some number of copies of Timely gave white decks a board presence and life buffer in the matchups where that matters most. With decks like U/W Control previously being pushed out of the format, even players like myself started to turn to Burn to prey on the unprepared. But with the return of Jace, the Mind Sculptor, and Teferi, Hero of Dominaria showing his power in Modern, U/W Control is back. So the question now goes beyond “is Timely Reinforcements worth a sideboard slot,” to: “Should Timely Reinforcements be a main-deck card?”
Against Burn, Timely Reinforcements is a no-brainer. Not only does it provide some number of blockers, but it can effectively counter multiple burn spells through the life gain. As long as you can play around Skullcrack, Timely Reinforcements is back-breaking here and perhaps your most powerful available weapon.
Against the other aggressive decks of the format, it’s a mixed bag. Against Humans and Hollow One, Timely Reinforcements isn’t your best possible card but it is definitely very good. The life buffer is nice and the blockers are key. Protecting your planeswalkers and setting up your sweepers allows you to punish these decks, but you need that time. Against Affinity, however, it has both the ability to kill you with infect damage and has a ton of evasive creatures, so the tokens are negligible.
Cards that are powerful against the most aggressive strategies tend to be the worst cards you can have against control decks. Most players assume that Timely Reinforcements will be one of the first cards you might sideboard out in a control mirror, but U/W master Gabriel Nassif suggests otherwise. With sweepers coming out in the matchup and many of the creatures being 1 toughness (like Snapcaster Mage and Vendilion Clique), the life gain is usually pretty irrelevant, but the pressure provided by some Soldier tokens is not. This is a great way to threaten an opposing Jace or Teferi, and the fact that Timely can be a solid card in these matchups is what sets it over the top for me.
Now, against midrange decks and combo decks, Timely is a pretty weak card. There are definitely games against midrange where a few tokens can provide chump blockers to let your planeswalkers take over, but that’s a passable effect and not a powerful one. Against combo, Timely often doesn’t do anything as they may not be dealing damage to you until they kill you. It’s definitely not what you’re looking to do.
But midrange decks are not a big part of the metagame, so having a mediocre card there isn’t a dealbreaker. A potential dead card against combo stinks, but that’s probably going to be the case anyway with your sweepers and weak main deck interaction. U/W has access to some of the best sideboard cards in all of Modern, so things change dramatically after game 1. Because the power level is so high against the popular aggressive strategies and still surprisingly good versus the many control decks in the format, I recommend main deck Timely Reinforcements for as long as the format allows it.