Gary Neville says his overriding concern from England's disappointing draw against Scotland was their lack of sharpness, and believes Harry Kane's performances can only improve once his team-mates show more intensity.
Kane's underwhelming performance in the stalemate on Friday followed a similar display in England's opening win over Croatia, both of which ended in the captain being substituted in the second half.
The striker denied a lack of fitness or his desire to leave Tottenham this summer were the reasons for his poor showings, and he was backed by Gareth Southgate on Sunday when the manager confirmed he would start England's final group game against the Czech Republic on Tuesday.
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Speaking to Sky Sports News, former England defender Gary Neville supported Southgate's decision, saying: "I said last week I felt he was undroppable and I still maintain that.
"I think that if Harry Kane had someone behind him who was even close to him that could go into the team, then you'd say maybe you could look at a change. But you wouldn't change Kane quickly, or easily for that matter.
"I think he'll break all England's records. You don't just drop your captain and your main goalscorer after two matches. But I think the energy, urgency and intensity in England's performance has got to improve for Kane to do well in this tournament.
"In the first game, we felt that was a controlled performance and everyone was really happy with that. But against Scotland, it was a massive drop off. I don't care who you are - as a centre-forward, you would have struggled in that game."
Reflecting on England's draw with Scotland - which ended with boos ringing around Wembley - Neville said: "The only thing that concerns me now is the physical levels, and I'm not sure why that occurred.
"It's one of two things: one, the players are very tired, and if they are then we've got a real problem. Or two, they were a little immature and mentally built the game up so much that they drained themselves.
"My concern was that it wasn't just the Harry Kane thing - the rest of them lacked sharpness. Even when [Jack] Grealish and [Marcus] Rashford came on, the performance didn't change.
"For me, going into the Czech Republic game - understanding that I have played with England teams in tournaments that looked fatigued - that is the only thing that I'm looking at in the first five, 10 minutes. Is the sharpness back, and was it just a mental thing going into that Scotland game?"
England know that they need to beat the Czech Republic at Wembley on Tuesday in order to top Group D, and Neville believes Southgate may make more changes to his defence for the game.
Luke Shaw and Reece James replaced Kieran Trippier and Kyle Walker against Scotland but failed to shine, leading some to suggest Southgate instructed his full-backs to rein in their attacking instincts.
However, Neville said: "This idea I've heard that Reece James and Luke Shaw didn't get forward because they were held back; I don't believe for one second that Gareth Southgate instructed James and Shaw not to cross the halfway line.
"They didn't look like they could go forward at speed, like we've seen them all year. If they were giving them that instruction, they would have picked Trippier and Walker. You're only picking James and Shaw to run forward quickly, put crosses in the box and join the attack.
"I definitely think Walker will come back in, and it wouldn't surprise me if Trippier came back in as well. What I don't think we can see is [Ben] Chilwell coming in and having three different left-backs in the first three games."
Southgate may also consider handing a first appearance of the tournament to Harry Maguire, who has not played since being injured in Manchester United's win at Aston Villa on May 9.
Neville described the Czech Republic game as "now or never" for the United captain, saying: "Now's the time for some continuity - we need to start settling on a team. That might mean that Harry Maguire comes back in.
"If he doesn't play tomorrow against the Czech Republic, I don't know how he can come in against a France or a Portugal in round two. It's now or never for Maguire."
What changes should Southgate make?
Sky Sports' Adam Bate
Having only changed his full-backs so far this tournament, it seems likely that Gareth Southgate's hope after beating Croatia was that another win against Scotland would allow him to rotate heavily against the Czech Republic with his knockout line-up already clear.
Such an unconvincing performance last time out complicates things. Southgate has already stated that Harry Kane will start again, an indication that the England manager is more concerned with rhythm than rest right now. As a result, major changes seem unlikely.
This writer would prefer more adventure in the full-back positions given the personnel available, but keeping the back four compact appears to be a key principle for Southgate. He even volunteered the comparison with France's defensive full-backs. It is the plan.
Abandoning it now would be an overreaction given that England are yet to concede a goal. Southgate is convinced that keeping things tight is the key to success in tournaments. On the other hand, this is the final opportunity for any experimentation. He needs to use it.
Expect him to use this game to gain a clearer understanding of where Harry Maguire and Jordan Henderson are with their fitness. A first competitive start for Jude Bellingham would be welcome. In attack, Jadon Sancho will need to play if he is to be a serious option during the knockout stages. Jack Grealish offers balance on the left.
That means benching players such as Raheem Sterling and Mason Mount who are sure to be recalled. But these selections would be a useful way of introducing players who might be needed later if England are to entertain serious hopes of winning this tournament.
England (4-3-3): Pickford; Walker, Stones, Maguire, Chilwell; Henderson, Rice, Bellingham; Sancho, Kane, Grealish.