The Spaniard's car was heavily damaged in the start accident at Spa, where Nico Hulkenberg crashed into him and launched him over Charles Leclerc's Sauber.

As a result, McLaren is short on spares of the latest parts used by Alonso.

There are also still doubts about whether his Renault engine survived the accident, and a call will be made after it runs on track on Friday.

"I think it's a chassis that I used in winter testing or the first races," Alonso said of the replacement ahead of the Italian Grand Prix. "So it's something that we already raced with.

"On the engine side it could be OK, but we will find out probably tomorrow in FP1.

"We want to put it tomorrow and confirm everything's OK. There were some concerns on Sunday, then Monday and Tuesday. They checked everything properly and it seems OK, so hopefully we can run still with that engine."

Asked about if he had enough examples of the latest spec parts, he said: "We have enough luckily, but probably we've run out of spare parts, we have just what we have on the car. All the rest will probably be a different specification.

"As I said, this is quite tough, because the damage on the cars was quite extreme, especially on my car. Then you have four days only to build a completely new car for Monza. It's the way it is."

Alonso admitted that the early retirement was doubly frustrating because McLaren had opted not to give him new engine elements and send him to the back of the grid - which was done with teammate Stoffel Vandoorne - so he still has to take a grid penalty at some stage.

"Because of the penalty of Hulkenberg and Bottas and Sainz we opted not to change the engine in Spa, and benefit from the P14 starting position, and then on the first corner you're out of the race.

"You will have to pay the penalty either in these race or later on, so it's a double zero – one in Spa because of another guy, and another one when you take the penalties. So quite a sad day."

Alonso left Spa on Sunday night with his right wrist bandaged, noting that he would know in a few hours if he had an issue - but he now says it is not a serious problem.

"Yes, it's fine. It was [felt] on Sunday a little bit, and I'm still feeling some kind of heat there, because I had the hands on the steering wheel when I touched Leclerc. Every day it's less and less, and it should be OK for tomorrow."

He also had some back issues earlier this week: "Honestly the only painful day was Tuesday. On Sunday it was fine, on Monday it was OK, but on Tuesday when I woke-up I had the back, I felt a little bit sore, and a little bit of a pain there.

"But yesterday was fine, today was fine, so it was all OK after that."