Shaw, who sustained a head knock while on England duty last weekend, sat out the 2-1 win at Watford despite being cleared to feature by United's medical staff.
Ashley Young deputised at left-back but will make way in midweek.
"Luke Shaw plays Wednesday [Thursday AEST]. We can't do it with one left-back and Ashley Young is not just a left-back, he's also a right-back," Mourinho said.
"They both are doing well, and they both are different players and I think they both understand that one is a young player and the other one is the experienced one.
"It's good for us to have these two options, plus [Matteo] Darmian. Darmian is my emergency man, he is always ready for everything."
Romelu Lukaku and Chris Smalling's first-half goals proved enough for United to bank three crucial points at Vicarage Road as it withstood a fightback after Andre Gray's 65th-minute strike.
David de Gea's late heroics made sure of the result, the goalkeeper denying Christian Kabasele's header after a free-kick in stoppage-time.
"Yes, only one save but a fantastic save," Mourinho said of the Spain international.
"In some matches you don't touch the ball, in some matches you are just somebody that is watching the game, but in the right moment you have to be there.
"Today his moment was that save in the last minutes, so good for him and for us."
The Portuguese, however, was not prepared to offer an opinion on the red card Nemanja Matic received for a second booking prior to De Gea's stop.
"They only thing I know is that [referee] Mike Dean was very good, the assistants were very good and [fourth official] Paul Tierney on the touchline was very good too," he said.
"It was an easy job for Paul, because me and Javi [Gracia] didn't create any problems, but they did a very good job and if it is a yellow or not, I don't know. It's too far for me [to see]."