The 19-year-old was a noticeable omission from the clash at St. James' Park, which United won 4-1.
"Mason wasn't fit to play, so we had to leave him out," Solskjaer said. "We hope he will recover quickly."
Dan James excelled in Greenwood's absence and Solskjaer was pleased with how the Welshman combined with his teammates.
"Attacking-wise, I thought Dan with the speed would create problems for teams and Juan and Bruno are intelligent and create chances for us and we could have scored a few more.
"It's not just about attackers though, it's about the team working for each other pressing from the front. Dan has incredible speed on the counter and today he showed more positivity, he trusted himself. We work with all the players to express themselves and show what they're good at."