Jansen indicated after Friday's game against the White Sox that he was unavailable to pitch due to a lat issue, Julian McWilliams of The Boston Globe reports.
The Red Sox had a save situation in Friday's game, but Josh Winckowski got the call in the ninth inning instead. Jansen didn't provide a specific timeline for his return, saying that he wouldn't pitch again until the issue is fully resolved. Winckowski appears to be the next man up as closer, but Chris Martin, Justin Slaten and even Liam Hendriks (elbow) might also be factors in the ninth depending on how long Jansen is unavailable.