The Spain forward signed for £21million from Barcelona on Thursday, with the Premier League champions swooping for a player United had pursued for around a month.
Mourinho was asked if the arrivals of Pedro and Baba Rahman, a left-back signed from German club Augsburg after last Sunday's 3-0 loss to Manchester City, provided a boost for his squad reeling from Chelsea's worst start in 17 years.
"Normally we train very well
This week we trained better than ever," Mourinho said ahead of Sunday's clash at West Brom.
"Why? Because we are not happy
I'm not happy with the results, the players are not happy.
"We want to do better and the only way is to work well
New players are always welcome so obviously I'm happy that both of them came to us.
"People used to call it the transfer window
I sometimes call it the opportunity window
While the window is open there is always an opportunity."
Mourinho insists Chelsea did not strike a blow to United by taking Pedro ahead of Louis van Gaal's side.
"I don't want to say we won, they lost, Man United wanted him, didn't want him," he added.
"The point is that Pedro is a player we like a lot and we are happy to have."
World Cup and European Championship winner Pedro was described as "one of the best attacking players in the world".
"Normally when you are (at Barcelona) you don't want to leave," Mourinho added.
"For many years he won everything playing with (Lionel) Messi, with (Andres) Iniesta, with all these stars, but he wants more.
"I think (it) is an evident sign of ambition."
Mourinho was not prepared to comment on his medical team after the controversy which erupted when first-team doctor Eva Carneiro and head physiotherapist Jon Fearn were removed from the bench ahead of the City contest.
He also refused to be drawn on whether Chelsea would return to bid for defender John Stones despite Everton's rebuttal of a reported three bids, the latest of which was said to be £30million.
Juventus' former Manchester United midfielder Paul Pogba has been linked with a £60million move to Stamford Bridge
Again, Mourinho refused to comment.
The Blues boss said: "I know that (Juve boss Massimiliano) Allegri was all summer speaking about Oscar and Ramires, but I don't do that with Juventus players."
But Mourinho said any opportunities which presented themselves before the transfer window closure on September 1 had to be assessed.
He added: "Can you do something that you really believe you cannot lose the opportunity to do?"
Any activity could have repercussions for Juan Cuadrado, Oscar, Ramires and Willian, but Mourinho gave short shrift to suggestions of departures.
Captain John Terry is likely to start in defence at West Brom despite being substituted at half-time at City, while goalkeeper Thibaut Courtois will return from suspension in place of Asmir Begovic.
Manuel Pellegrini became the latest manager with a surname beginning with the letter P to beat Mourinho's Chelsea - Tony Pulis has won his last two encounters against Mourinho while at Crystal Palace and, in May, at West Brom.
"I wouldn't mind to go to the superstitious side of the game," Mourinho said.
"I never win against a P and seasons where I start with a 2-2 at home I win the treble.
"2-2 Porto-Belenenses, we won the treble (in 2004), 2-2 Inter-Bari, we won the treble (in 2010)
2-2 against Swansea?"
Source : PA
Source: PA