Manchester United team struggled against Swiss side Young Boys on Tuesday and looked to be heading for a second successive 0-0 draw before Fellaini saved their blushes in stoppage time to secure a 1-0 win.
Victory propelled them into the round of 16 with a match to spare after next month’s opponents Valencia lost at Juventus in the other Group H match.
“I think the mood is good,” Fellaini said. “The players are happy but the most important thing now is to focus on Saturday and three points.
“It’s always difficult to come back from the national team. Now we have a full week of training. All the players are ready to compete and start Saturday.
“Criticism is part of football but we have to work. We are a team, we are together, and give everything too. Together we have to move forward and take the points.”
United, who travel to Southampton on Saturday, are languishing in seventh place in the Premier League, seven points behind fourth-placed Chelsea, and are desperate to close that gap. Defender Chris Smalling suggested a lack of confidence played its part on Tuesday.
“We played well against them away but we knew that at home they were going to be hard to break down,” he said of a side United managed to beat 3-0 in Switzerland.
“I think we created a lot of chances and it was deserved but they made us work for that. We created some good chances – not just half chances – and I think heads can dip when a few of those don’t go in.