Rafa Benitez has given a fitness update ahead of Newcastle United's game against Southampton.
The two relegation-threatened clubs meet at St James's Park tomorrow.
Midfielder Shelvey missed last weekend's game against Liverpool with a knee problem, while striker Slimani (thigh) is yet to make his debut.
Slimani joined the club on loan from Leicester City in January.
"Jonjo's been training, but we have to be careful with him," said United manager Benitez. "We have to decide for tomorrow."
The Southampton game is followed by a 21-day break.
Newcastle will return to action on March 31, when Huddersfield Town visit St James's Park.
"I was talking with (Slimani) two or three days ago and half an hour ago," said Benitez. "He's improving and doing well.
"I don't want to say the Huddersfield game (for his debut), but in terms of time, maybe."