The 24-year-old played a huge part in helping the Glovers reach the Championship but departed to Millwall on transfer deadline day after 148 appearances across three-and-a-half years in Somerset.
And Johnson explained the thinking behind the departure of a player who was criticised by his manager when he gave away a 'stupid free-kick' in his final match for the club against Nottingham Forest.
"You never want to lose one of your better players but over the past couple of months he hasn't been one of our better players and for whatever reason he hasn't come up to the standard of what we hoped he would in the Championship," he told the Western Gazette.
"He wasn't in the team and was out of contract in the summer. He's a fantastic person but when we got the offer it was an offer we should take with the circumstances."