Mourinho said that Peter Bosz's side should have been eliminated from European football entirely this season, once they were knocked out of the Champions League at the group stage.
"Fosu-Mensah plays Sunday [against Crystal Palace] and if the reaction is good and he plays without problems, he is a new option for us for the final".
UEFA Europa League finalists Ajax and Manchester United will carry out media activities at the Friends Arena on Tuesday 23 May for accredited personnel only.
The Portuguese youngster, who begun this season on loan at Primera Liga side Belenenses, was brought back to Old Trafford in January and handed his first senior appearance against Wigan Athletic in the FA Cup.
But when asked if the young players were ready, Mourinho said: "All together? No".
"The only positive is to save my players to give them better conditions for Wednesday", Mourinho said.
The Red Devils have struggled to produce consistent form in the Premier League and they are now sixth in the table heading into the final round of games.
"I don't know [Ajax boss Peter Bosz] too well, but when I see Ajax play I think he has done fantastic work". One in the middle of a good structure, an experienced structure, I think yes. They trained with me many times.
"Obviously there is a big difference of quality, strength, experience between them and good players, experienced players like Crystal Palace have. The one I really enjoy to do is the transition from the first to the second season".
It has robbed the 35-year-old Swede of a possible United swansong and the chance to write the ideal script in his homeland when United face his old club Ajax in Stockholm, though Mourinho has revealed Ibrahimovic will travel to Sweden to be at Wednesday's game anyway. "Strong guy, strong mentality".
Zlatan Ibrahimovic, Marcos Rojo (both knee), Luke Shaw (foot) and Ashley Young (hamstring) are still nursing injuries and are set to miss the rest of the season. "I don't know", Mourinho said on Rooney.
Mourinho could not resist another mention of the fixture schedule United have faced, insisting observers are not doing enough to emphasise how hard their calendar has been.