Summer interest in the 25-year-old turned into an active pursuit last month, with protracted negotiations with Sporting Lisbon finally bearing fruit towards the end of the transfer window.
Fernandes arrived for an initial 55 million euros (£46.6million) and, while that could rise to 80 million euros (£67.7million) if a variety of targets are reached, United look to have struck a good deal.
It was a far better performance from United than the one at Vicarage Road two months ago, when Solskjaer's side fell to a 2-0 loss as Nigel Pearson started life at the helm with a bang.
Watford were far from that standard on Sunday, although they saw a Troy Deeney equaliser ruled out as Craig Dawson was adjudged by the video assistant referee to have handled.
"It's a frustrating day in the sense that, when you play the bigger clubs, it's very important that when you create chances or have opportunities, you do everything you can to take them," the head coach said.
"We didn't take the chances we had early in the game. I think our attempts to press them high up the pitch caused them a few problems in terms of frustrating them at times."