"I was disappointed by that because, knowing the individuals, they are both highly thought of. They've got personal problems which I don't need to relay," PFA Chief Executive Gordon Taylor told Sky Sports.
"Sometimes a manager needs to be a psychologist as well, and also to be a counsellor because you can't treat everybody in the team (the same), every manager must know that. Every player in a team is different."
Smalling has missed several United matches with a foot injury but took a pain-killing injection to play against Chelsea last month, according to British media reports.
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Shaw, who recently returned following a 10-month injury layoff, has had a mixed start to the campaign and lost his place in the starting line-up after Mourinho publicly criticised the 21-year-old following United's loss at Watford in September.
"It's a short-term career and we're all human beings. I think that's what management these days has to be," Taylor added.
"I'm not talking about being soft, I'm talking about being understanding because to even get to the top in football you have got to succeed as not so many actually make it in the game."
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by John O'Brien)
