After three bad years with Honda, the British team has finally dropped the Japanese manufacturer and switched to Renault power, but McLaren still had winter testing trouble.
Alonso told Spanish radio Onda Cero: "I do not see any reason to be pessimistic.
"McLaren is the second best team in history, Renault is one of the best engine makers ever and we are full of ambition. I do not see why it will not turn out well."
However, the team had higher hopes at the launch of the new orange car, refusing even to rule out wins.
"Things have not changed much since the presentation or the plans we had for the winter," Alonso insisted.
"We wanted to improve on the last three years and try to get as close as possible to the three best teams. We are on the way to doing that."
Alonso admitted that McLaren will not join the fight with Mercedes, Red Bull and Ferrari in 2018, though.
"McLaren can jump from being ninth to being close to them and fighting in the race," said Alonso.
"When we changed to Renault we all agreed that it would be good if McLaren was close to the top three, in the top five in every race possible, always in Q3 and fighting for a podium when we can."
Alonso denied that McLaren dumped Honda at the worst possible moment, with Toro Rosso apparently performing well with the Japanese engine in the winter.
"I have no problem with Honda," said the Spaniard. "It never crossed my mind to leave.
"But we will see how the Honda is in Australia. They did a good winter test and if they are in the first five I will applaud. But I don't think so. One test is not the same as 21 races with three engines.
"I think they can be in the last three or four positions and we at McLaren will try to be in the first five, six.
"We are very happy with the change to Renault - we have improved four seconds compared to last year."
Alonso said that he never considered leaving McLaren, but he admits that 2015-2017 were "difficult to cope with".
"My life is based on competition and winning," he said.
"When I lack victory it is hard but I also learned other things. In these years my sights were opened up to other legendary competitions like Indy and the WEC. Everything happens for a reason.
"Those years had some frustration but now I'm happy because I have become a more complete driver."
As for the likely 2018 champion, Alonso agrees that Mercedes "seems to be in total domination mode".
"Normally there are six cars in the first three grid rows, so then there is the question: McLaren, Haas, Renault or Force India?" he said.
"The first thing we have to do is to win that battle."
The first grand prix of 2018 takes place in Melbourne next weekend.