The team were knocked out of the Champions League last week, leading to fresh criticism of manager Louis van Gaal and reports that senior figures in the dressing room had questioned the Dutchman's methods.
"There's a great atmosphere. The dressing room is very together and united. They are a great set of lads and we're all very happy with the group of players that we have," De Gea was quoted as saying by the club's website (www.manutd.com).
"I think that's really important in a team if it wants to achieve big things.
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"This league is very tough to win and there will be a number of teams who will be in the shake-up for the title.
"That's what we all want, to stay up near the top, and to get to April and be in a position where we can win the league. That's what we're going to try to make happen."
The stopper also praised the influence of Rooney on the team's attitude.
United's skipper has missed the last three games in all competitions with an ankle injury but De Gea said his importance to the team went beyond his contributions on the pitch.
"As our captain he has a big personality. He always tries to motivate us. When things are going badly it's always him who speaks up and encourages us," the keeper said.
"He's breaking all kinds of records for goals scored and games played and I reckon he deserves it as he is such a great player.
"And he's the same off the field. He's the club captain and he's the one who has to do the talking and motivate us. That's what he always does."
United host 18th-placed Norwich City in the league on Saturday.
(Reporting by Simon Jennings in Bengaluru, editing by Nick Mulvenney)
