Greenwood did not travel with his team-mates to the north-east and missed out as United won 4-1.
Daniel James was recalled with Anthony Martial having been suspended and Edinson Cavani in quarantine.
It is not known what Greenwood’s problem is though the Red Devils are hoping for a quick recovery as they start their Champions League campaign against PSG this week.
"Mason wasn't fit to play, so we had to leave him out," Solskjaer said after the victory at Newcastle. "We hope he will recover quickly.
"Attacking-wise, I thought Dan with the speed would create problems for teams and Juan [Mata] and Bruno [Fernandes] 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.”
Harry Maguire, Bruno Fernandes, Aaron Wan-Bissaka and Marcus Rashford were all on the scoresheet against Newcastle.
United had to recover from an early Luke Shaw own goal with Solskjaer enjoying the reaction from his side.
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“[The early goal] made it a lot harder for everyone,” Solskjaer added. “It was hard enough as it was with the build-up to this game and the result from last time and then going 1-0 down after a minute and a half.
"I thought the boys responded fantastically. They were patient, resilient, they trusted what the plan was and in the end we created lots of chances to grind [out] a win.”