A dislocated shoulder is a relatively minor injury, so the charge will probably be 'ABH' rather than the more serious 'GBH' or the lesser 'Common Assault'.
If no weapon was used, and the court doesn't regard the assault as pre-meditated, the 'starting point' sentence for a first time offender convicted after a trial is a 'high' community order, with a 'sentencing range' (which the court would normally be expected to adhere to) running from a 'medium' community order up to 26 weeks custody.
However, if the court takes the view that their was pre-meditation involved in the offence (but still with no weapon involved and resulting in relatively minor, non-permanent injuries), the 'starting point' goes up to 24 weeks custody, with a sentencing range of 12 to 36 weeks custody.
As indicated, those sentences apply to a conviction after a trial. An early guilty plea would see a reduction in the sentence.
In practice, the courts usually try to keep first-time offenders out of prison whenever possible. The most likely sentence is probably a community order (perhaps with a requirement of, say, 200 hours unpaid work).
Chris