Somerside was appointed last week to succeed George Norrie, who left Garstang after only eight games.
It is a quick return for Somerside, who quit as first team coach at the end of
last season along with manager Gary Hargreaves after the pair had guided the club to record breaking achievements, including a promotion and the West Lancashire League title in their debut season in the Premier Division.
Somerside said he had planned to take a rest from the game for at least a year, adding: "I needed to re-charge my batteries, relax and wind down.''
But when he got the call last week from Garstang chairman Adrian Wilding, he says didn't hesitate to say yes.
"It did come a bit out of the blue and was unexpected but it wasn't a difficult decision for me,'' continued Somerside.
"I was still in touch with one or two of the players and I was aware that things were not 100% so when the call came I was only too delighted to go back.
"Injuries seem to have been the main problem and George (Norrie) has obviously suffered because of that, but I think there has been a hangover from the success we had last season and maybe the players didn't react well to the change when me and Gary left.
"Now I want to but the smiles back on a few faces at Garstang.''
Caretaker manager Dave Porter was due to take charge for Saturday's trip to Crooklands Casuals and will remain as assistant manager. Somerside was also planning to travel with the team.
"The majority of the side is the same as at the end of last season, so I know what I'm getting and when everyone is fit I know this is a strong group of players,'' said Somerside.
Norrie left following the 2-0 defeat at Vickerstown, but club officials said the decision to part company was due to the appointment not "working out in the interests of the club'' rather than results.
A club spokesman said they were delighted to have re-recruited Somerside.
"As soon as we knew that Scott had thrown his hat in the ring to be considered for the position, the committee were unanimous that we need not look any where else,'' he said.
Chairman Adrian Wilding added: "We are delighted that Scott has returned to the club as manager. We need stability throughout the club. Work going on off the field must be complemented by matters surrounding the playing side.''