McGrath wrote for the Sunday World: "The way he handled Callum Hudson-Odoi last week, bringing him on and then taking him off 20 minutes later, before picking him to start against Atletico, impressed me.
"It sent a message to the heart of the Chelsea squad – 'Being a good footballer just doesn't cut it now, you have got to work for the team in the way I want you to.'
"Hudson-Odoi clearly got the message, because, once forgiven by the boss, he played very well against the Spaniards.
"And I suspect the message got through to others, too.
"Maybe today, though, we might see what Tuchel can bring to the party from a tactical side of things.
"I wonder would he put a 'sitter' on Bruno Fernandes to stop United's Portuguese star having such a big influence on the match?
"And the tightest defences, at the biggest clubs, limit what even the most gifted modern-attacking player can do.
"Now 13 points adrift of Manchester City, United simply have to win this afternoon's game to keep the title race alive before the teams clash at the Etihad next Sunday.
"Chelsea will want to win it also, to keep their Tuchel-inspired run up the table going so that a fourth-place finish at the end of the season remains a realistic ambition."