This team is a contender. The determining factor will be their ability to respond to the offensive droughts this team suffers through. If the team can respond by going up another level defensively or consistently getting to the line or consistently hitting big shots, this team can actually win it.
During the exhaustion that is May/June Playoff basketball, defenses collapse on post-ups, shut down driving lanes, and stick to shooters. The teams that have elite mid-range and/or FTA specialist as one of their best players will have an advantage over their opposition (considering the teams are relatively equal) because these are the shots that defenses do not impact with the same effectiveness.
The Bucks, Raptors, and the Sixers lack of mid-range games will make those teams vulnerable in the 4thQ (though they have elite FTA specialist). The Celtics have 4 players that have solid mid-range games, if one of them can level-up to elite, it lifts the ceiling to Championship contender. Without that, and teams being evenly matched, styles will make fights.