Rafa Benitez admitted he was left "counting players" after losing two more defenders to injury.
Newcastle United's season took another turn for the worse at the St Mary's Stadium this afternoon.
Benitez's second-bottom side were beaten 3-1 in the Premier League by Southampton.
Taylor and Janmaat – who made mistakes for the first-half goals scored by Shane Long and Graziano Pelle – were forced off.
Janmaat limped off after slipping in the build-up to Pelle's goal, while Taylor was replaced by Jamaal Lascelles at the break, having suffered an injury during the an abject first 45 minutes.
Asked about their injuries, Newcastle manager Benitez said: "I don't know. We will see.
"We have had a lot of them (injuries). We have to see how they are, and after start counting players for the next one."
Taylor mistimed a challenge on Long for Southampton's first goal, scored in the fourth minute.
Asked about Taylor, Benitez said: “He was a little stiff.
"So I think it was a big risk to keep a player with problems on the pitch, especially with the problems we have in defence, so that is why we took Steven off.”
Benitez has a week to assess his squad before United take on Swansea City at St James's Park on April 16.
Fit-again defender Paul Dummett was on the bench at the St Mary's Stadium, while captain Fabricio Coloccini, sidelined since February, is close to a return.