Hernandez is back on the training ground after being sidelined with a groin problem since coming off in the 2-1 win over Southampton on November 6 and, if all goes well, he could be involved in the televised game at the Riverside Stadium.
Hull will go into the game with just one fully fit and available striker in the shape of Adama Diomande with Dieumerci Mbokani suspended and Will Keane out for the season with a serious knee injury.
But despite a serious dearth of forward options, Phelan insists he will not be tempted to take a risk with Hernandez.
He says he will not selecte him until he is convinced of his fitness.
Phelan said: “He’s been with the conditioning guys for a few days. We will have a look at him and then reassess come the weekend to see where he is for the Middlesbrough game.
“We are unsure right now. Any player that can be out on the training ground, I expect them to be in a state to train.
“(But) we wouldn’t take risks and I have not done that with the likes of Andy Robertson and players before him.
“We have options. I have a good young player in Jarrod Bowen who could play that role. I have got Diomande, who is a forward. I have got others who can play outside of their normal positions.
“There are things we can do. We could go with no centre forward. But we will come up with something between now and Monday.”
Meanwhile, Phelan says he has been inundated with messages after Hull were paired with Manchester United in the last-four of the EFL Cup early in the New Year with the Tigers chief to make a sentimental return to Old Trafford for the first leg in the week beginning January 9.
Phelan, on the coaching staff at United for 14 years during Sir Alex Ferguson’s reign, including five years as his assistant – and also an ex-player – said: “I’ve had a few text messages, mainly from the press guys.
“To have gone out at this stage would have left us with a few sour faces and a few down in the dumps, but now it’s given everyone the opportunity to still work hard through Christmas and push on from there.”