The Spaniard signed from Villarreal in the summer, the club splashing out a reported £24m snare the playmaker on a five-year-deal.
Fornals arrived in England fresh from starring for the Spanish Under-21s and the deal was brought about by director of football Mario Husillos, who managed his progress through El Submarino Amarillo's academy.
After some encouraging signs early on, the 23-year-old grabbed his first goal for the club in August's 2-0 EFL Cup win at Newport.
However, since then he has struggled to impose himself on the team but Pellegrini has kept faith and started him in Saturday's 2-0 loss at Everton.
Bernard and Gylfi Sigurdsson were the scorers for the Toffees who dominated proceedings at Goodison Park to end a run of four straight Premier League defeats.
Like many of his team-mates, Fornals struggled to have any impact on the match, appearing to be ruffled by Everton's physical pressing game before being subbed off after 63 minutes.
Pellegrini now looks to have a decision to make on whether the Castellon-born maestro gets the nod on Saturday at home to Sheffield United.
After the defeat at Goodison, and going down to Crystal Palace before the international break, the Hammers will be keen to get back on track and the boss may look to select a more robust side against the Blades.
For now, he seems happy with his Spanish starlet and says he always expected Fornals to take time to settle into English football.
"It is normal for a player that comes from another league to have some months to adapt to the different kind of games, different pace from the Spanish league," he told the club's website.
"I am happy with him. As a manager, I am regretting missing the three points against Crystal Palace at home more."