The former Bristol City left-back, who is yet to feature this season having suffered from a knee problem, played 45 minutes of the behind-closed-doors clash before limping off at half-time.
"I think Macca's struggling a little bit, he's had a bit of a relapse of his injury and it doesn't feel quite right," Johnson told This Is Somerset.
"That's why we need these games because we can't afford to do that in proper games."
The opening goal in the 2-0 win was a stunning free-kick from Republic of Ireland Under-21 international Conor Henderson, currently on trial with the club.
Johnson added: "You can see that he's not quite fit just yet so we're really just having a look at him because we were asked to have a look at him.
"He's been a top player, of course, but he's had a cruciate ligament injury, which is a big injury, and we're just taking a look.
"We gave him an hour and you could see the Arsenal in him. Of course, he's a long way away from match fitness at the moment."