Mick Harford has quit as Colchester assistant boss and joined the coaching staff at QPR, reports BBC Essex.
Harford was appointed by U's manager Geraint Williams
Harford said the commute from his Hertfordshire home to north Essex played a big part in his decision to become John Gregory's number two.
"It is a real wrench to leave but one of the main reasons is my health," the former Luton and Chelsea striker said.
"I had an operation to clear-up a couple of problems and I was told the 170 mile round-trip wasn't helping."
Harford added: "In fact the 30-odd hours I'm spending behind the wheel are a major factor in the deterioration in my back.
"I was offered a very attractive new contract to stay and we had talked about my spending more time in Colchester but I'm very settled where I live with plenty of friends and family."
Harford, 48, joined the U's at the start of last season and helped steer the club to 10th place in their first Championship campaign.
QPR's previous number two, Richard Hill, was suspended indefinitely by the club following February's infamous brawl with the China Olympic team.
Early favourites to replace Harford are the club's player-coach Aidan Davison and former player and present Charlton reserve-team manager Mark Kinsella.