The teenage striker was sent home from the squad along with Phil Foden after the pair broke coronavirus protocols in Iceland, although the Manchester United boss believes Greenwood shouldn't have been there in the first place.
Speaking ahead of United's Premier League opener against Crystal Palace, the Norwegian coach said: "The kid had a fantastic season.
"He's just come through. I've worked and the club have worked and all the staff here, we worked really hard, when we gave Mason enough time on the pitch, when we gave him exposure in the media.
"Then you come to the end of the season and we've had less than two weeks off, and he's called up to the national team."
Solskjaer added he was keen for the youngster to have a break during the brief close season and vowed to protect the forward following the incident.
He explained: "I have to be honest enough to say I tried my best to give him a rest over the summer.
"The club asked specifically - we pointed out he should have had a rest. He needs in mentally and physically after that season he had. First thing that happens is, one he's called up, two he's right there in the press.
"We've done whatever we can to protect him and I will keep on helping him. I know that when he comes back into the daily routine of the club and what we do to him - he'll be fine."