Can a player be part of a team's backroom staff during a tournament?

In England, as part of the build-up to the Euro 2020 (2021) tournament, there has been much discussion between fans about the inclusion of both Jordan Henderson and Harry Maguire – who have either just returned from injury and lack any real match fitness (JH), or are currently suffering an injury (HM) – in the final 26-man playing squad for the tournament.

When questioned on the inclusion of these two players, the England manager Gareth Southgate responded:

I think [Maguire is] such a good player and we had the additional size of squad, the fact that that leadership which is growing in him all the time

[…]

They both have a bit to do to be available but we feel the possibility they can play a part and the fact they are such fantastic characters to have around the group

[…]

Hendo, his leadership and presence is already having an affect on people, especially the younger ones in the group but all the others as well.

^{Source – Evening Standard}

It is clear that Southgate sees both these players as key leadership roles in the squad (both are also captains of their respective domestic teams), and as such deems them a calculated risk to include them into the squad.

But given that taking injured players, or out of fitness players, to a tournament is a risk – especially when there are deemed to be in-form replacements available – are there any rules which prohibit the manager from taking players as part of the "backroom staff", as opposed to them being in the playing squad?

Obviously JH’s/HM’s leadership skills would be better served if they were on the pitch, but given the apparent importance of leadership to Southgate, is there anything stopping him from keeping them around the playing squad, for their leadership, without actually being part of it throughout the tournament?