Fearn was criticised alongside then first-team doctor Carneiro for going on to the pitch to treat Eden Hazard during the 2-2 draw with Swansea on the opening day of the season on August 8
The pair were subsequently stripped of first-team duties.
Carneiro did not appear on the bench again and left Chelsea the following month
The 42-year-old is pursuing a claim of constructive dismissal against the club and a separate personal legal action against Mourinho.
Fearn remained at Chelsea, working behind the scenes.
Chelsea have long insisted the cases were separate, with Mourinho intimating Fearn could return to the bench prior to his sacking as manager in December
And now he has returned for Saturday's Premier League clash with West Ham at Stamford Bridge.
Fearn, who was on the pitch for the warm-up, replaced Steve Hughes on the Chelsea bench for Saturday's match, for which Hazard was absent with a hip and groin injury
Carneiro and Fearn's treatment of the Belgium international against Swansea more than seven months ago meant Chelsea were temporarily down to nine men, having already had Thibaut Courtois sent off
Afterwards Mourinho called Carneiro and Fearn ''impulsive and naive''.
Chelsea, Mourinho and Carneiro are scheduled to appear before an employment tribunal on June 6, unless a settlement can be reached beforehand.
Should the cases proceed to an employment tribunal, witness statements and documents - including texts and emails - would be made public, while Carneiro, Mourinho and representatives from Chelsea could be called to appear as witnesses.
That could be difficult for Mourinho, who is keen to return to management and has been linked with a move to Manchester United, with Louis van Gaal's position under scrutiny.
Chelsea wore black armbands for another member of the club's medical department, after practice manager Pamela Jordan died following an illness
Source : PA
Source: PA