"I can't thank Ernie enough. It is all down to him that I am now as fit as I am. When he told me to lose 10kg I suggested five. He said okay - then lose another five.
"Thinking back, what was I doing playing football at 101kg? Now I'm down to 90kg I feel so much better and am more mobile, fitter and stronger.''
Watson joined Lower Hutt City as a four-year-old and was with the club until he joined the Phoenix's school of excellence and played for Team Wellington.
Merrick said the surprise move in the last hours of the transfer window was to secure cover for the strikers.
"We have wide players in Kenny Cunningham, Roy Krishna and Tyler Boyd, but needed cover for our central high strikers Jeremy Brockie and Stein Huysegems,'' he said.
"We had a scare with Stein's calf last week and if one of the two strikers got injured we would have been a bit bare.''
The eighth-placed Phoenix meet Newcastle across the Tasman on Sunday.