The result means United can finish no higher than sixth with one game remaining and must rely on winning the Europa League final against Ajax to seecure a Champions League berth.
"He may be a player one day but right now as we sit here after one year in our football, I'm still waiting to be convinced".
The likes of Paul Pogba, Marcus Rashford and Ander Herrera were rested, but there was still a multitude of talent on display at St Mary's.
United manager Mourinho didn't reveal the nature of Fellaini's injury, but he suggested the problem wasn't too serious.
He said: "United have to play a premium". "Because it's Man United and they have the money".
He is finally in a position to begin his preparations for next season, when he hopes to avoid another battle against relegation, and discussing how much Sunday's game could affect that, he said: "The budget will get bigger if we can go up a couple of places".
"(Daley) Blind played today after 90 minutes on Thursday.
The game had little significance for either side, with United guaranteed to finish in sixth place and Southampton now in eighth place and unable to finish any higher.
"Any team has a debrief at the end of the season and this is normal but the end of the season is after the last game", said Puel, 55. If I go for example to Rostov, the way he played in Rostov was awesome.
James Rodriguez, Antoine Griezmann and Romelu Lukaku are rumoured to be transfer targets of the Red Devils but according to reports in England all of them could snub a move to the Old Trafford in case the Red Devils don't play in Champions League next season. "I hope next season some club beats the record with another player and then the pressure goes somewhere else".
Mourinho has rotated players in Premier League matches in recent weeks as he has plotted his way through the Europa League. "They showed me they want to be an option for the final".
'These are examples of trying not to accumulate when people have to play because we do not have other players and I cannot put four or five kids all together in the fire.