This is a nonsense. Saints played 4-2-3-1 (one upfront) for the majority of the 2014/15 & 2015/16 seasons, scored a lot of goals and finished 7th & 6th in the Premier League.
Playing one upfront is fine... if the manager sets the players out with instructions and mentality to support the lone front man well. That is the issue with Pellegrino, not there only being one striker.