The influential duo missed the defeat at Derby last Saturday after lateral flow tests showed they had tested positive for Covid-19.
A subsequent PCR test, however, showed Grant had actually recorded a ‘false positive’ with the striker then given the green light to report for training a few days later.
Mowatt, meanwhile, had contracted Covid-19 but the midfielder was asymptomatic and will have completed his isolation period in time for the visit of the Bluebirds.
“KG (Grant) received a negative PCR test after the (Derby) game,” head coach Valerien Ismael said.
“He missed the game due to a wrong positive test, lateral flow.
“He has trained since midweek completely, with the squad.
“Mowatt will be available for the game on Sunday.”
Albion’s performance at Derby last weekend was their worst of the season – with Ismael fuming with his players at full-time.
The Baggies entered the game with a chance of jumping into the automatic promotion places. But they underperformed throughout with Ismael going on to question the mentality of his players.
Since then the head coach has held some honest and frank conversations with his squad.
And while he has been pleased with their response so far, he says they now have to show the right mentality when it matters.
“We will see on Sunday – but so far it’s been good,” Ismael said when asked how his players have responded to the poor showing at Pride Park.
“We’ve had open discussions with the players.
“We said a lot of things, my thoughts.
“I have been here six months now, I have got an overview on everything, we’ve given almost everyone a chance to play and to show their quality.
“It’s decision time now, with the support from the January window, it’s time now to show up and see who is able to follow our way and who is not.”
Albion’s defeat at Derby – and draw at Barnsley in their previous outing – has led to third-placed Blackburn moving four points above them in the table.
Meanwhile, fifth-placed Middlesbrough are just two points behind.
Ismael, though, isn’t concerned by Albion’s current position in the table.
“It’s the Championship –you will see, the crazy time will come,” the boss said.
“If you see the schedule, February, March, it’s crazy.
“A lot of teams have missed a lot of games that they’ll need to play.
“So things will change – this is my experience.
“We just need to stay focused on us, on the next game and start winning. It’s clear for everyone, we must start winning with consistency if we want to reach our purpose.”