Fellaini did not feature in Belgium’s international friendly against the Netherlands on Tuesday, a match that finished 1-1.
Martinez revealed that the former Everton midfielder was not “100 per cent” physically ready for that clash, sparking fears that he could miss out at the weekend, but the Spaniard is confident he will prove his fitness.
“He should be fine, it's just he wasn't 100 per cent,” he said.
“He's got an important game at the weekend and this wasn't the game to use players who weren't 100 per cent.
“We expect he should be fine in the next couple of days.”
As Jose Mourinho attempts to rebuild United’s season following a slow start, Fellaini has been an important figure for him. Indeed, the Belgian has started the club’s last five Premier League matches, while he also netted a dramatic late equaliser against Derby in their third-round Carabao Cup clash, though the Red Devils ultimately lost that on penalties.
Mourinho’s side go into the weekend’s match against the Blues eighth in the Premier League standings, having suffered three defeats already this season, though their come-from-behind victory against Newcastle at Old Trafford a fortnight ago has temporarily eased the pressure upon the embattled Portuguese.
A loss against Maurizio Sarri’s unbeaten Chelsea, however, would leave United 10 points off the early-season pace and would renew scrutiny over Mourinho’s long-term future at the club.