There is no requirement to have 11 fielders on the field at all times. Law 1 (The players) makes provision for the number of players per team to be varied by agreement of the captains:
By agreement a match may be played between sides of fewer than, or more than, eleven players, but not more than eleven players may field at any time.
A no-ball will not be called if a fielder leaves the field or fails to takes the field. Law 2.1 (Substitutes and runners) specifically states that a fielder may leave the field, thereby leaving fewer than 11 fielders on the field:
A player wishing to change his shirt, boots, etc. shall leave the field to do so. No substitute shall be allowed for him.
However, according to Law 2.5 (Fielder absent or leaving the field), she/he may not return without the consent of the umpire (which will be given as soon as possible). If she/he does so and touches the ball while in play, the umpire will award penalty runs under Law 2.6 (Player returning without permission):
If a player comes on to the field of play in contravention of 5(b) above and comes into contact with the ball while it is in play [...] the ball shall immediately become dead and the umpire shall award 5 penalty runs to the batting side.