Welbeck, who joined from Brighton earlier in the summer, trained alone on Thursday, raising some doubt over his fitness ahead of what would be his competitive debut for the club at Craven Cottage. Alonso played down any concern.
"He can be involved," Alonso said. "He needs a couple of days not training too much because of a little thing. We expect him to be back on Saturday."
A number of Chelsea players had been given extended breaks after reaching the latter stages of this summer's World Cup, among them Reece James, Enzo Fernandez, Maxence Lacroix, Morgan Rogers, Valentin Barco and Malo Gusto, who only linked up with the squad following the club's pre-season tour of southeast Asia.
James, Rogers and Lacroix all started in Chelsea's final warm-up match against Real Sociedad last week, with Gusto and Fernandez introduced from the bench.
Alonso confirmed all of the returning group are in contention for Monday's trip to west London rivals Fulham.
"Let's see, we will check, we will assess," he said. "For sure, because of the World Cup they might need a little bit more time. They are selectable for the line-up."
The Spaniard also gave an encouraging update on Levi Colwill, who tore his ACL almost exactly a year ago and missed the majority of last season as a result.
"I would say having a proper pre-season has helped him in taking the right steps in the right moment," Alonso said. "We don't want to rush him too quickly because of what he had last year. Now, after pre-season, we are expecting him to play in a good way with the confidence and the fitness to have him at his best level. For me, he is one of the best centre-backs in the world."
Chelsea rounded off their six-match pre-season schedule with a 3-1 win over Real Sociedad at Stamford Bridge, giving Alonso a full week at the club's Cobham base to finalise his plans before the trip to Craven Cottage.
The Spaniard has had around six weeks to work with the majority of his squad since taking charge in the summer, as Chelsea look to build on last season's 10th-place finish and push back into Champions League contention.
Chelsea and Fulham are the last two Premier League clubs to open their campaigns this weekend, meaning Alonso has the luxury of an extra few days on the training pitch compared to most of his top-flight counterparts before his side get their season under way.