"He's very good", Mourinho said.
A sluggish Manchester United side finally overcame Hull thanks to a late victor from youngster Marcus Rashford.
United striker Zlatan Ibrahimovic had earlier troubled the home team but was flagged offside when he back-heeled the ball into the side netting.
Keane, twin brother of Burnley centre-back and fellow United academy graduate Michael Keane, would have had to fend off competition from Zlatan Ibrahimovic, Wayne Rooney, Anthony Martial and Marcus Rashford to break into the Red Devils' match day squad - let alone the starting line-up.
Mourinho told the press on Friday: "I prefer it this way because to be selected (for the seniors) and be on the bench behind (Harry) Kane and (Jamie) Vardy and all the experienced players, and play one minute or not play".
Rashford struck a 92nd-minute victor, having replaced Juan Mata 20 minutes earlier, as United broke Hull's resolve to extend Mourinho's winning start at United.
"We have to sit down, me and the owners, sometime".
"Of course, I feel sorry for Hull but my guys deserved it and the way the fans supported them, especially in the second half, the fans had that feeling too and knew we were doing everything".
"People will judge me on the goals I score, so that is what I want to do". "It was a big moment for him to go to the Euros and it's still important for his development to go and play with the Under-21s". It's a respectful dialogue.
"But I will certainly not stop playing football", he added. "I still believe in my own ability".
"This Man United team now is on a bit of a roll and they are strong and strong in the depth they have got as well".
The Tigers, who are one of the favourites for relegation, had a surprising start to their season when they beat Leicester City and Swansea in their opening two games before narrowly losing to Manchester United in their third game.
Phelan is desperate to bolster his squad before the close of the transfer window with Hull yet to make a new signing so far this summer. That's why it's not possible now.
The striker made his United debut in a 3-2 home defeat against Blackburn Rovers in the Premier League nearly five years ago.