The five-time Ballon d'Or winner, who left United to join Real Madrid in 2009, returned to the club where he made his name as a global star last month from Juventus.
Since joining United, Ronaldo broke the all-time international goalscoring record with his 110th goal for Portugal in a World Cup qualifier win against the Republic of Ireland. Having being suspended for the subsequent game against Azerbaijan for removing his shirt in celebration, he jetted to England and has been training with his new team-mates for the past few days.
Ronaldo is joined by United's other summer signings Raphael Varane and Jadon Sancho in a star-studded team also including Paul Pogba and Bruno Fernandes. "He's been having a good pre-season with Juventus, he's played for the national team, he's had a good week with us here," Solskjaer said.
Ronaldo won three Premier League titles and the Champions League during six trophy-laden years with United, before going on to shine with Real and Juventus.
It seemed United would miss out on a reunion with their former star when Manchester City moved to sign him. But Ronaldo has said his relationship with former United manager Alex Ferguson played a key role in convincing him to return to Old Trafford, calling the move "a dream come true".
Solskjaer, who played with Ronaldo during his first spell at the club, said: "Of course we've followed his career from afar since he left here and I think everyone's very happy to have him back. He can speak for himself but he seems like he's happy to be back as well. The mood's been very good."
Before the unexpected acquisition of Ronaldo, Solskjaer had already strengthened his squad with swoops for Sancho and French World Cup winner Varane. United will be expected to challenge champions Manchester City for the title after such a productive transfer window. Solskjaer believes Ronaldo will add a higher level of quality to the squad, both physically and mentally, as they try to land a first trophy of the Norwegian's reign.
"We have grown as a group the last few years. The determination, the desire, the focus has improved over time and bit by bit," he said. "Even like bringing Raphael (Varane) in, he's come in with a winner's mentality, an unbelievable focus. Then of course Cristiano comes in as well and there's no place to hide when you've got winners like them. You can't go into training and give 95 percent or not being focused."
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