The Blues 2-1 defeat by United last weekend left Sir Alex Ferguson's side needing just one point from their final two games to scure a record 19th English title.
Chelsea, bankrolled by Russian billionaire Roman Abramovich, spent £50 million signing striker Fernando Torres from Liverpool and £25m on centre-back David Luiz during the January transfer window.
But the London club, who won the domestic double last season under Carlo Ancelotti, are set to end this campaign without a trophy after losing in the FA Cup to Everton and being knocked out of the Champions League by United.
"I always think we should strengthen all over the pitch (in the summer)," England skipper Terry told Chelsea TV.
"I look back when Jose Mourinho was here - we had a very big squad and great players who could come in, but then that brings financial problems to the club and players become unhappy.
"We literally had 11-13 players who weren't involved, but they were a very good set of quality players and we have missed that.
"We need somewhere in between where we are now and what we had back then.
"We lost a lot big players this year and maybe we needed a bigger squad, but that's no fault of Roman or this football club.
"What he has done for Chelsea, I couldn't put into words. It's very easy for us to say we'd love to see this or that. It is his money and he has dedicated a lot of time and effort, but I would say we need players."
Central defender Terry felt defeats by lower in the table teams such as Wolves, Sunderland and Birmingham, did huge damage to Chelsea's title hopes.
"When it comes to the big games, nothing has been too different in that the players have been very well prepared and gone into them feeling like we could win them like last year.
"Last season we won at Arsenal and at Old Trafford quite comfortably, but I always say they are very good sides, especially at home, and you can win or lose those games.
"The matches against the other sides were important, the Wolves game stands out for me and Newcastle, those are the ones you have to win because when you're playing those kind of sides, the other clubs are playing similar teams and winning those matches.
"If you drop points there it makes things more difficult and puts pressure on the bigger games."
