One of the pleasant surprises for the Boston Celtics during the NBA has been center Robert Williams, who has become a key staple in Brad Stevens' rotations.

However, Williams played just 15 minutes during Boston's 100-93 loss to the Toronto Raptors in Game 4 of their Eastern Conference semifinal on Sunday.

Stevens said he elected to go with smaller lineups in part because Raptors big man Serge Ibaka posed a problem for the Celtics' team defense:

Ibaka has emerged as a major offensive threat for the Raptors in the past few seasons, improving his perimeter shooting and becoming a legitimate pick-and-pop option also capable of diving to the rim for floaters and layups.

The veteran dissected the Celtics on Saturday, scoring 18 points while making all four of his three-point attempts. Apparently, the ability to switch only diverted Boston's problems in the second half, as Pascal Siakam scored 11 points and grabbed four rebounds in the third period.

It will be interesting to see whether Stevens changes up once again for Game 5. Or, perhaps he will mix and match personnel. Semi Ojeleye played just 11 minutes on Saturday, but he scored seven points and grabbed three rebounds, finishing plus-six for the game.

Maybe more Ojeleye and Williams will give the Celtics added size and interior defense. Regardless, the Celtics could probably stand for more aggressiveness from Kemba Walker.

Kemba had success operating in pick-and-roll with Daniel Theis early in the series, choosing when to attack or swing the ball back to Theis, who often made the extra pass to find an open shooter. But Walker was more passive on Saturday, taking just nine shots.

In any case, the Celtics are left searching for answers after the Raptors tied the series at two games apiece.