Roma appear vulnerable as they enter the last 16 of the Champions League nearly a year on from their incredible run to the semi-finals, where Liverpool finally eliminated them.
Porto await, quietly confident of landing themselves a quarter-final spot with one of the easiest routes in living memory, having gone unbeaten in a group including Schalke, Lokomotiv Moscow and Galatasaray.
The Giallorossi have one of the youngest, most talented sides around, including midfield revelation Nicolo Zaniolo, can they hold their composure at home to take a lead back to the Dragao? Follow it live:
"Over 180 minutes I would say it's 50-50," Tuchel said. "Tomorrow, with them at home at Old Trafford whether it's 50-50 I'm not sure but we have a second leg in Paris where we are strong and comfortable.
"At this level it's the details. It could be a decision of a referee, a yellow or a red that can make the difference. It's a close game.
"They play with a lot of confidence, they score a lot of goals and get a lot of early leads," he said. "It's an impressive run. To play at Old Trafford in this moment against this Manchester United is one of the toughest challenges you can have."
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“We know Anthony’s quality going forward and hope he’s going to create chances for us from the left-hand side and show us what he can do. We have to be clinical, of course, and it’s important we get to Paris with a chance to go through,” Solskjaer told BT Sport.
“They’ve done well together in a few games," he said of Victor Lindelof and Eric Bailly's partnership in defence. "Of course, against [Kylian] Mbappe, they need to be on top of their game,” said Solskjaer.
Roma head coach Eusebio Di Francesco says his side must not let Porto score what could be a vital away goal in tonight's first-leg.
"We have a great opportunity to produce a good performance and bring back some enthusiasm," said the 49-year-old.
"It's vital that we don't concede, although we can't allow that desire to compromise our identity and the way we want to play."
Porto manager Sergio Conceicao, meanwhile, returns to Stadio Olimpico having represented Lazio during his playing days.
"Obviously entering this stadium comes to memory the happy years and the many titles I have conquered in Lazio," he said. "It's always an emotion to come to Rome, but my focus is tomorrow's game.
"There is no special feeling for confronting Roma. There is a special feeling for having at the heart the symbol of FC Porto.
"Every day I feel a great pride in representing a club as big as ours."
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