His uneasy relationship with the Old Trafford fanbase was stretched to breaking point once more following his senseless sending off against Manchester City two weeks ago.
Sergio Aguero made a meal of the contact, but it was hard to find reasoning and mitigation for Fellaini ducking his head into the Argentinian.
It is for this kind of brainless incident that explains why Fellaini remains a hugely divisive figure and may forever remain that way.
Back in December, Jose was forced to leap to the Belgian's defence again after he clumsily conceded a penalty at Everton four minutes after coming on.
United squandered two vital points and Fellaini was booed by sections of the crowd when he came on against Tottenham the following week.
Fellaini remains a exasperating mystery of contradictions. His clumsiness and heavy handed approach is baffling and at times nasty - yet three United managers have trusted him and there can be no denying his importance in this side.
Indeed, there is still the sense that he remains an unsophisticated Plan B, the shambolic transfer strategy of David Moyes, and the epitome of the floundering post-Sir Alex regime.
Jose started Fellaini in the first five league matches of the season, before Ander Herrera became the regular man in the deeper midfield role.
Yet the Belgian has fought back into the reckoning, operating higher up the pitch and offering an outlet, a physical presence and a goalscoring threat.
Mourinho has been impressed by Fellaini's work rate and willingness, and he continues to grow in stature and credibility as a United player.
Fellaini has a knack of influencing important matches for the team and rose to the occasion once more against Celta Vigo.
He scored in a Manchester derby under Louis van Gaal and his 17th-minute header against the Spanish side was his fourth of the season and third in semi finals.
He scored the opener in United's 2-1 FA Cup win over Everton at Wembley last term as United went on to lift the trophy, and played a pivotal part in Juan Mata's leveller in the final with Palace.
Fellaini scored the crucial, tie-settling second at Old Trafford in January's EFL Cup semi final tie with Hull and was at it again on Thursday.
His far-post header, which has become something of a trademark, made the decisive difference as United were left frantically clinging to that goal in the dying moments.
Fellaini deserves credit for turning his Old Trafford career around, if only he can cut out his stupid and reckless off-the-ball errors.